Under the rule of Augustus the Empire achieved relative stability, but the weakness of some Imperial successors led to the emergence of pretenders supported by provincial armies. In the early centuries AD, Roman military power both preserved the Empire – and divided it.

■ Ancient Authorities

For the early part of the long period which we are now considering, many writers already mentioned provide valuable testimony. Indeed, they often have greater value to the extent that they are dealing with subject matter nearer to their own times.

The system of imperial government inaugurated by Augustus (as Octavian called himself from 27 BC onwards) meant that strict impartiality could be a dangerous virtue in a historian of contemporary events. On the other hand, emperors were often happy to be compared with their predecessors to the detriment of the latter. Handsome tribute to the Emperor Nerva thus provided Tacitus with a pretext for unflattering portraits of earlier emperors – an indirect outlet for his Republican sentiments. Such sentiments were common among the literary men of his time. Virgil, Horace and Livy had cherished no illusions.

Cornelius Tacitus was born about AD 56. The exact date of his death is unknown, but he was consul in 97 and proconsul in 112–113. Coming from one who held high administrative posts, his evidence on contemporary events is naturally of historical importance. Tacitus’ two major works, which have partly survived on the basis of tenuous manuscript tradition, are known as the Histories (probably covering in their complete form the period AD 69–96) and the Annals, which must originally have spanned the whole period from the death of Augustus in AD 14 to that of Nero in 68. Of the Histories, the first four books and part of a fifth are extant. Of the 16 books of the Annals which evidently existed, Books VII–X are lost, while V, VI and XVI are incomplete.

Tacitus also wrote a monograph on Germany and the German peoples, an ethnic study tinged with admiration for primitive virtues. But more important for our theme is his Agricola. Gnaeus Julius Agricola enjoyed a distinguished military and administrative career in Britain, having served as tribune in the army of Suetonius Paulinus, who crushed the revolt of Boadicea (Boudica) in AD 60. It is fortunate for students of history that Tacitus was Agricola’s son-in-law.

Contemporary with Tacitus, there lived a remarkable historian with a very different background. Flavius Josephus, born AD 37, was a patriotic Jew who, though opposed in principle to the act of rebellion, himself commanded Jewish resistance forces against the future Emperor Vespasian in AD 67. When captured, he was first spared, then patronized by Vespasian, whose rise to imperial power (in AD 69) he had happily prophesied. Apart from an account of the war in which he himself was involved, preceded as it is by a lengthy introduction, Josephus wrote, originally in Aramaic and available to us in its Greek edition, the history of his own country. He began his history with the creation of the world and tried to place Biblical and Graeco-Roman history in context with each other. In this endeavour, he was later followed by Christian writers including Eusebius, Bishop of Caesarea in 313, whose Ecclesiastical History and Chronicon contain matter which adds to our knowledge of the Roman Empire in the first three centuries AD.

The historians Zosimus and Aurelius Victor wrote after the adoption of Christianity as an imperial religion, but they are pagan in outlook and sympathy. Although their authority, particularly that of Zosimus, for fourth-century events is more valuable, we owe to them summary accounts of the pre-Christian Empire: notably the resurgence of Roman military strength under Claudius Gothicus and Aurelian.

The autocratic system of government naturally led to the equation of history with imperial biography. Unfortunately, for many periods during the third century AD, we are obliged to rely heavily on the Historia Augusta, a name given by the French scholar Isaac Casaubon (1559–1614) to a collection of Roman imperial biographies relating to the years between 117 and 218. The six historians who contribute to this record are uncritical in their approach, and the documents on which they purport to base themselves are often to be considered suspect.

In prefacing a chapter which concerns itself at last with events AD, it is perhaps pertinent here to notice a chronological anomaly. Herod, the Idumaean (Edomite) king, whom Mark Antony, after the Parthian invasion of Syria and Palestine, established as ruler of Judacea, died 749 years after the foundation of Rome. In the sixth century, the Christian abbot Dionysius Exiguus assigned the birth of Christ to the 753rd year after the foundation of Rome; and the years of our era are numbered on the assumption that he was correct. However, the gospels make it plain that Christ was born shortly before the death of Herod, in 4 BC (or slightly earlier) according to the accepted Dionysian reckoning.

■ Political and Military Considerations

Augustus was able to establish the authoritarian regime on which Roman maintenance of widespread law and order depended, largely because he had assumed power while young and lived to be nearly 77 years old. Longevity is at any time a matter of some luck. In Roman political circles, whether of the Republican or Imperial epoch, it was a matter of great luck. The unity and continuity provided by a single head of state, exercising uninterrupted power for 44 years, was in fact fortunate for the whole Roman world.

Augustus never contemplated abolishing the time-honoured magistracies of the Republic. He simply assumed all the key titles himself: consul, tribune, proconsul and, after the death of his old triumviral colleague Lepidus, Pontifex Maximus (Supreme Pontiff). He called himself Princeps – a word which in its most general sense meant “leading man”. This was in addition to his other constitutional title of princeps senatus or “leading senator”. He presided over an exhausted world, which had reluctantly realized that law and order can be worth more than liberty, and that authority was destined in the foreseeable future to be based on military power, whatever constitutional forms were adopted. Julius Caesar had shown more respect for constitutional appearances in his last years, as a dictator, than in his early years as a demagogue. It may have been memories of his early career rather than the conduct of his late life that exacerbated Republican sentiment and brought about his murder. Augustus was at all events at great pains to preserve the outward forms of a constitution.

The real source of his power was not merely the army, which now accepted his unrivalled supremacy. From the days which had immediately followed his great-uncle’s death, he had realized the political importance of finance. After Philippi, lack of funds had considerably embarrassed him, but with the downfall of Cleopatra, the vast treasury of Egypt, which for different reasons Cassius and Antony had both failed to commandeer, was at his disposal. His “privy purse” (fiscus) was administered separately from the Roman state treasury (aerarium), but in practice he controlled both funds. Similarly, there was a distinction between imperial and constitutional procedure in provincial administration. The outlying frontier provinces, in which the Roman legions were stationed, were more obviously under command of the emperor; in home territories, where war was not expected, administration was more apparently civil and senatorial.

If we wish to stress the constitutional aspect of Augustus’ rule, we may refer to it as the Principate, but the term Empire is that which has best survived in history, the word “emperor” being derived from imperator, the title by which a victorious general had normally been acclaimed by the celebrating populace in the later days of the Republic.

Apart from the legions in military provinces at the circumference of the Roman world, it was important to the emperors that they should be able to rely on a nucleus of armed strength at the centre. The Republican general’s unit of headquarter troops, the praetorian cohort, developed in Imperial times into the Praetorian Guard, a privileged corps d’élite. The Guard, quartered in the vicinity of the city, was originally composed of nine cohorts, each probably 1,000 strong, and included both infantry and cavalry elements. These served as a bodyguard to the emperors.

In 2 BC, two officers (praefecti) were appointed to command the praetorian cohorts, and as praetorian prefect, Sejanus (Lucius Aelius Seianus), the adviser of Tiberius, Augustus’ successor, attained dangerous power. Later, the Praetorians realized only too well the extent of the emperors’ reliance on them. They became makers and breakers of emperors.

Three urban cohorts were also created for police purposes in the city. They served under their own prefect and were each commanded by a tribune. In practice, their political significance became comparable to that of the Praetorians, though they were not paid so highly.

In addition, there were seven cohorts called cohortes vigilum, who served as firemen and night police. Other troops regularly stationed in Italy were the marines at the naval bases of Misenum and Ravenna. These contingents were sometimes used for fatigue and pioneer duties in support of the army or in aid of public works.

■ The Frontiers of Empire

The Roman navy, at such times as it could be said to exist at all, was always the junior service. However, Augustus was at pains to maintain it, for he needed to preserve lines of communication between Italy and the provinces. Of no small account were the naval forces whose allegiances had been transferred to him after the defeat of Antony and Cleopatra, and he was able to establish fleets in the eastern and western Mediterranean and in the Black Sea. Other naval squadrons operated on the Danube, the Rhine and in the English Channel.

Campaigns in Illyricum, under Augustus’ destined successor, Tiberius, had safeguarded the route to the east by the Via Egnatia and Thessalonica, and the freedom of the Adriatic from pirates was further assured by the construction of the naval base at Ravenna. The Mediterranean in general was well policed under Augustus, and his was the last Roman administration to take effective measures against piracy.

Preoccupation with sea routes was the logical accompaniment of provincial road-building which proceeded under the Empire. Italy in the time of the Republic had acquired a good road system. Apart from that, the Via Egnatia, referred to above, and the Via Domitia, which led from the Rhône to the Pyrenees, were also Republican achievements. In Augustus’ time, new Alpine roads were made and communications facilitated with the Danube. The characteristically straight Roman roads, adhering where possible to high ground, were planned to satisfy military requirements. But at the same time, of course, they opened the way to trade and assisted official contacts.

The legions which in the first century AD extended and, later, defended the frontiers of the Empire were distinguished by names and numbers, though some of the numbers were duplicated. The names commemorated the patrons or creators of the legions, as for example the Legio Angusta, or else they referred to some event in regimental history, or marked a local connection, as in Macedonica or Gallica. Augustus’ army originally contained 28 legions. But three of these were annihilated in the great Roman military disaster of AD 9, when Augustus’ general, Publius Quinctilius Varus, was treacherously ambushed by the German chief Arminius in the Teutoburgian Forest. The numbers of these three ill-starred legions were as a consequence never allotted to Roman legions at a later date.

A Roman governor, in charge of an imperial province, ordinarily ranked as a legatos of the emperor. Legions apart auxiliary troops including cavalry contingents were an important element in the garrison of a province. Under Augustus, auxiliaries, which during the first century BC had been composed of foreign troops, once more began to recruit Roman citizens. This was in part because Roman citizenship itself had by now been conferred on many communities and individuals outside Italy. The social distinction being lost, auxiliaries tended to be integrated with legions. In permanent frontier stations auxiliary cavalry and infantry were posted at first from distant provinces. But as a matter of convenience, auxiliaries came to be recruited locally and the distinction between the legionaries and auxilia was accordingly once more obscured. However, military policy favoured independent cavalry tactics. From the reign of Trajan onwards, tribal non-Romanized units, known as numeri, were recruited; their role corresponded in some ways to that of auxiliain more ancient times.

The disaster which the Romans suffered in Germany under Varus was the result of an attempt to establish frontiers farther east, on the Elbe. Its effect was that Roman emperors were from that time onward content, as Julius Caesar had been, to rely on punitive and retaliatory action in order to assert a Roman presence on the Rhine. Augustus himself, at the end of his life, made it quite clear that his territorial ambitions were not unlimited. Defence, however, often entails offensive initiative, and he had been at great pains to secure the line of the Danube.

The most suitable location of frontiers was a question which left room for uncertainty, above all in the reign of an emperor of unbalanced mind, such as Gaius (Caligula) proved to be. His inexplicable vacillations could well have been damaging to Roman prestige, and the expansionist policies of the mild-mannered Claudius, who succeeded him, may have been necessary to ensure that enemies beyond the frontier were left with no illusions about the reality of Roman strength. Claudius, in need of a military reputation, added first Mauretania, then Britain to the Empire. Roman domination was carried farther by Trajan, who annexed Armenia and temporarily occupied much of Parthia. Rome, however, was never able to impose itself finally on the Parthians.

■ Armed Insurrections

Apart from frontier fighting, Roman forces were on various occasions during the first century called upon to deal with local rebellions. Information at our disposal is often meagre, but such insurrections seem to have been variously motivated. It is not always easy to distinguish between local grievances and national aspirations of the peoples involved. Rebellion might naturally be expected in a recently subdued province such as Britain. The tribe of Catuvellauni, perhaps subjects of the Cassivellaunus dynasty which had confronted Julius Caesar, had by now extended its sway over south-east Britain. A refugee British prince, seeking aid against his father Cunobelinus (Cymbeline) had given the Emperor Gaius pretext for an invasion, but Gaius contented himself with a military demonstration on the Channel coast of Gaul. When a similar opportunity presented itself to the succeeding Emperor Claudius, it was taken in earnest. Caratacus,¹ the British prince with whom the Romans now had to contend, was defeated. He took refuge with Cartimandua, a northern British queen, who was a Roman ally, but she betrayed him and he was sent with his family as a captive to Rome. Claudius magnanimously spared his life.

Boadicea (Boudicca), who revolted in the year 60, was not a member of the Cassivellaunus dynasty, but had been left queen of the Iceni at the death of her husband. The harsh and humiliating treatment of Roman administrators drove her to take up arms. The governor of Britain, Gaius Suetonius Paulinus, who had played a prominent part in the conquest of Mauretania nearly 20 years earlier, hurried back from the uncivilized regions of north-west Wales where he had been operating, and Boudica’s defeat and suicide followed. Meanwhile, however, Camulodunum (Colchester), London and Verulamium (St Albans) had been sacked with a massive death toll among the Romans and their British adherents.

Gnaeus Julius Agricola – even allowing for the fact that Tacitus was his son-in – law-must have been an able and energetic administrator. He had served under Paulinus in Britain, and in other parts of the Empire as well, before his appointment as governor in 78. His military operations did much to secure Roman rule in Britain. Tacitus would have been a better military historian if he had paid closer attention to geography. He probably had no precise idea himself as to where the battle of Mons Graupius was fought or where the river Tanaus was. We are left to guess that the former site was somewhere in Scotland, and the Tanaus has been identified variously as the Tyne, Tweed, Tay or even Solway. Fortunately, archaeology has come to our aid in the tracing of Agricola’s movements. At all events, he carried his victorious campaigns into the Scottish Highlands. As a demonstration of strength, he circumnavigated the whole island of Britain, and his military successes were accompanied by wise administration.

The conflict with Boudica may be traced in part to extortionate Roman financial practices: such in fact as had, under the Republic, rendered the eastern Mediterranean world sympathetic to Mithridates. Roman rapacity was a recurrent source of trouble and had in AD 21 led to a notable insurrection in Gaul, led by Julius Sacrovir. Sacrovir was a Romanized Gallic noble of the tribe of the Aedui, Julius Caesar’s old allies. He was eventually defeated by the Roman governor of the upper Rhine province (Germania Superior) and committed suicide. But the same record of local misgovernment continued to explain revolt. Administrators like Agricola were the exception rather than the rule. Later in the century (89), an upper Rhine governor himself, Lucius Antonius Saturninus, raised an insurrection and caused the Emperor Domitian to set out in alarm on a northwards expedition. The governor of the lower Rhine, however, remained loyal to the Emperor and Saturninus was defeated and killed, his eastward German allies having been prevented, apparently by a sudden thaw, from crossing the frozen river. The precise occasion of this revolt is uncertain, but again one would guess that finance lay at the heart of the matter.

Among serious rebellions that occurred in the first century of imperial history was the Jewish War, in which Josephus was involved. This was the product not only of economic causes but of outraged religious susceptibilities. The Romans were in general tolerant of religion, but did not know how to deal with religions which were themselves intolerant (as Judaism and Christianity were). The violence did not end when, after a horrifying siege, Jerusalem fell (AD 70) to the future Emperor Titus. Its total destruction, accompanied by appalling loss of life, provoked a series of revolts by Jewish populations in other provinces, which culminated (115–116) in insurrections throughout Syria, Cyprus, Egypt and Cyrenaica. Casualties are reported as running into hundreds of thousands. These events in turn led ultimately to repercussions in Palestine. Of Bar Kokhba’s revolt, something has been learnt from the literature of the Dead Sea caves.

■ The Events of AD 69

Insurrections during the first century of imperial government suggest that there was often enough provincial discontent to provide ambitious leaders with a cause. They also show that there was sufficient military ability available to provide aggrieved communities with effective leadership. This was all the more inevitable in view of the fusion between Roman and local elements in the army. Sacrovir and Saturninus, and even Arminius, the destroyer of Varus’ legions, were or had been Roman officers. It was only a matter of time before provincial rebellions could aim at the overthrow of the emperor himself and at the transfer of his power into hands of the rebels’ choosing.

The events of the year 69 can be described more accurately in terms of civil war than of revolt, but they are the logical outcome of military and political precedents. A rebellious Romanized Gallic governor (Julius Vindex) had been defeated and killed by troops from the adjacent upper Rhine province. However, Sulpicius Galba, whose specious claims to restore the Republic he had supported, was in command of legions in Spain, and when the Praetorians at Rome proclaimed Galba as emperor, Nero, last of Augustus’ dynasty, tearfully committed suicide. Galba was soon installed at Rome, but his nomination of a successor disappointed one of his military adherents, Marcus Salvius Otho, who now conspired with the Praetorian Guard. Nor did Otho long enjoy the fruits of Galba’s murder, for a secret, as Tacitus observes, had been revealed by Galba’s shortlived success: emperors could be created elsewhere than in Rome. Even the encouragement of the Praetorians was no longer necessary.

Otho was challenged by Aulus Vitellius, a Rhine commander. The legions of the eastern and Danube frontiers apparently supported Otho. But even if the eastern support had been sincere, it was distant, and the Danube troops were slow to move. Vitellius’ officers won the crucial battle for him near Cremona, and he himself, after Otho’s suicide, travelled to Rome at his convenience. The eastern legions, however, now showed their hand and proclaimed as emperor the 60-year-old general Vespasian (Titus Flavius Vespasianus). Vespasian was in a position to block the Alexandrian corn supply. But apart from that, the Danube troops favoured him. Italy was invaded. Fighting again converged on Cremona. Vitellius was prevented by his own supporters from coming to terms with Vespasian’s brother in Rome, and the latter was killed in the fighting which followed. However, the victors of Cremona soon arrived at Rome. Vitellius was hunted down and dragged to his wretched death.

Vitellius had disbanded the Praetorian Guard and replaced them with Rhineland legionaries of his own. As such, they naturally supported him, but they were unable to resist the wrath of the invading provincial legions. The situation in which the legions of one province were not in accord with those of another may from this time on be regarded as familiar. To all appearances, conditions which had prevailed in the last century of the Republic had now been recreated. Until Nero’s death, dynastic prestige had secured continuity of government, but it needed a leader of exceptional ability to establish a new dynasty. Fortunately, Vespasian was such a leader. He reigned ten years and died at the age of 70. His sons Titus (Titus Flavius Vespasianus) and Domitian (Titus Flavius Domitianus) succeeded him in turn as Emperor.

Vespasian’s accession to power was marked by complications in Gaul. Julius Civilis, who commanded Batavian auxiliaries recruited from the Rhine delta, had on the request of Vespasian’s officer in Italy created a diversion hostile to Vitellius. This provided Civilis with the opportunity for an independent uprising. He allied himself with a nationalist movement in Gaul, which purported to set up a Gallic Empire in place of the apparently crumbling Roman authority. The Gallic movement soon collapsed, but there was a military lesson to be learnt. Roman army units composed of foreign nationals under their own leaders could easily become an embarrassment. A trend in future policy was to post foreign troops at a distance from their home territories and to arrange where possible that auxiliary units should contain more than a single nationality. As for Civilis, we do not know what happened to him. Our manuscript sources break off at this point, tantalizingly leaving him still negotiating with an eloquent Roman commander.

■ The Stabilization of Frontiers

The murder of Domitian in the year AD 96 was the outcome of domestic discord. Nevertheless, it gave great public satisfaction. Apart from his other shortcomings, the tyrant had failed to make adequate arrangements for a successor. The Senate appointed a newprinceps, Marcus Cocceius Nerva, and Tacitus was pleased to see in this constitutional gesture a revival of Republican sentiment. Nerva was an old man at the time of his elevation. He was also childless, and after one year of power he appointed a loyal and able officer, Marcus Ulpius Traianus (Trajan), as his colleague and successor. The appointment was timely, for Nerva died early in the following year. Under Trajan, imperial expansion was renewed, and as one of Rome’s greatest soldier emperors, he was shrewd enough to nominate an equally great successor. The formal nomination and adoption which usually secured the imperial succession was much more satisfactory than the common hereditary process. It generally ensured that the successor would be a military commander, for with exceptions, one of which we have just recorded, none but a soldier could hope to survive. The Empire depended for defence and government upon military force. As for the principle of adoption itself, Roman reverence for legal forms lent it all the sanctity of a blood-tie. One may compare the relationship of patron and protégé (cliens), which we have already had occasion to notice.

Hadrian (Publius Aelius Hadrianus) who, as a connection by marriage, was Trajan’s ward and became emperor on his death, in many ways reversed the policies of his predecessor. But this does not prove that either he or Trajan was wrong. Times were changing. The steady westward migration of peoples in Asia and Europe meant that pressure on Rome’s frontiers was steadily mounting. Under Trajan, those frontiers had attained unprecedentedly wide dimensions. Hadrian saw the need for contraction and consolidation, and this policy was marked in vulnerable areas by the construction of fixed fortifications, signal posts and entrenchments. A line of forts linked by palisades, protected the intrusive salient of territory between the upper reaches of the Rhine and the Danube. Hadrian’s name is notably associated with the Roman frontier works across north Britain from the Tyne to the Solway. The line of forts and base camps, connected by a mural barrier, replaced an earlier linked chain of forts slightly to the south. “Hadrian’s Wall” was initiated as the result of the Emperor’s visit to Britain in AD 122; Hadrian spent a great deal of his reign in visiting outlying provinces. The Wall exemplifies the principles of Roman frontier defence as they existed in many sectors of the Empire. A chain of strong-points was connected by a well-defined communicating road (limes) along which troops could move with efficiency and speed.

Antoninus Pius (138–161), who succeeded Hadrian, presided over an epoch of comparative peace and plenty in the Mediterranean core of the Empire. But the price of social well-being was continual vigilance and preparedness on the frontiers. In Britain, Antoninus tried to advance the frontier – as he did in Germany – and built another wall in the form of a turf embankment on a cobblestone base, farther north, from the Forth to the Clyde. But the time came when this could no longer be defended, and after only 23 years it was decided to withdraw southwards once more and rely solely on Hadrian’s stone structure for the defence of Roman Britain.

The recourse to engineering skills in order to solve manpower problems had been Julius Caesar’s answer. Rome’s wars against the barbarians were a continual struggle against numerical odds, and with the help of technology the Romans strove to make good what they lacked in numbers. Twenty-eight legions had been all too few for Augustus’ original ambitions, and when he lost three of them in Varus’ disaster, he saw the need to reduce military commitments and shorten the perimeter of the imperial frontiers.

The military garrisons which manned frontier areas were (as a matter of policy on which we have already commented) not all nationally homogeneous. But they tended to form settled communities as a result of relationships with local women, and the resulting settled habits and lack of mobility in themselves constituted a disadvantage. However, legions were withdrawn from Britain at various dates during the centuries of Roman rule, to meet pressures in other parts of the Empire, and such withdrawals, even though the legions by this time were not all first-line troops, opened the way inevitably to northern or seaborne invaders to make incursions.

■ The Task of Marcus Aurelius

Marcus Aurelius Antoninus, who succeeded to the principate at the death of Antoninus Pius in AD 161, was also of a quiet and philosophic disposition, but unlike his predecessor he was faced with the necessity for continual warfare. The fact that he was able to meet the challenge of military duty with energy and unbroken resolve indicates some kind of spiritual triumph over his natural temperament, at the same time making him a practising as distinct from a purely academic philosopher.

War against Parthia (162–3) was only a prelude to barbarian incursions on the Danube front (166). It was already well recognized that responsibility for imperial defence was more than a single emperor could support. An emperor’s nominated successor, who now ordinarily received the title of “Caesar”, was also a colleague. Marcus Aurelius was not very fortunate in his colleague Lucius Verus, whose adoption derived from a decision of Hadrian. Marcus, showing perhaps poor judgment of character, arranged that the task of imperial government should be shared, and Verus, ruling as an equal on a collegiate basis, took command of the war against Parthia, which was won for him by his able officer Avidius Cassius.

The major cities of Parthia were captured, but this victory, like that of Trajan, though westward territories were annexed, could not lead to permanent Roman occupation of Parthia. The days were past when Romans and Parthians fought each other with characteristic national weapons and battles were a conflict of highly disciplined legionaries with incalculable swarms of mounted bowmen. Arrian, writing on military tactics in the time of Hadrian, testifies to the diversification of arms and armour and the variety of combatant methods employed by the Roman army at that epoch. Trajan’s Column and other monuments tell the same story. The Romans had among their own contingents heavily mailed horsemen on the Parthian model; nor did they lack archers who could retaliate against the Parthians. If they were never able to bring the Parthian Empire within the bounds of their own, this was probably because they lacked sufficient troops to hold what had been conquered. Such vast deserts were in any case ungovernable.

Lack of numbers also told heavily against Roman defence on the Danube, and it should be stressed that Rome was now seriously on the defensive in this area. Various barbarian tribes, forced westwards and southwards by migratory pressures, crossed the Alps and reached Aquileia at the northern extremity of the Adriatic Sea. Italy was threatened as it never had been since the days of the Cimbric invasion, but the barbarians did not capture Aquileia, lacking the equipment for assaults on fortified towns. Marcus Aurelius, despite the inferior ability of his colleague, was well served by his generals on the Danube front. Lucius Verus in any case died on active service in 169, and Marcus was left in sole command.

There seems to have been a good deal of collaboration between the German tribes of the upper Danube and the Sarmatians farther east. Roman armies, relying simply on mobility and speed, had to turn abruptly from one threat to another. The invaders were defeated in a series of arduous campaigns, forced back across the Danube and reduced to quiescence. But such warfare spelt an end to current methods of frontier defence and, in years which followed, Roman strategists had to think increasingly in terms of fortified zones rather than defensible lines.

Unfortunately, the manpower problem in the time of Marcus Aurelius became all the more critical on account of a devastating plague which the army brought back from its eastern wars. Sheer lack of manpower obliged Marcus to establish a German militia, settled within the imperial frontiers, as a way of combating German threats from without. Military service was the price of the land which the settlers occupied. As the frontiers became less distinct, so also did the definition of Roman nationality. The operations of Marcus Aurelius and his officers secured the line of the Danube, but in the large frontier province of Dacia to the north of the river, which Trajan had previously annexed, a right of way was granted to the barbarian tribes, allowing them to preserve communications with their eastward compatriots. In some sense, the Empire was now provided with insulating zones but – to press the metaphor – this insulation could become a semiconductor of extraneous forces.

Marcus Aurelius would probably have rendered the territory beyond the Danube more secure, but in AD 175 he had to meet the revolt of his eastern deputy Avidius Cassius. It would seem that Cassius had been deceived by a false report that Marcus was dead, and his dissident action hardly had time to gather impetus before he was murdered by one of his own centurions. Avidius Cassius would in any case have been a preferable alternative to the Emperor’s ineffective son Commodus, who eventually filled the role of official colleague and successor.

■ Septimius Severus and his Army

The principate of Commodus lasted 12 years, which should have been long enough to secure the succession, but Commodus did not allow the matter to trouble him. He was eventually murderer as the result of a conspiracy hatched by his Praetorian Guard commander, who had for some time shared the real power with other favourites, and at last decided that the present emperor was no longer necessary. During the next year, two emperors were proclaimed and then murdered, while the Praetorians tried to make up their minds. At last, they gave support to Septimius Severus, who commanded the Danube legions. The legions themselves, in fact, provided a firmer backing than Praetorian caprice.

Septimius had to fight for the imperial throne against other contenders, who were also supported by provincial armies. He was victorious in the ensuing struggle, partly because he commanded more troops than his adversaries and partly because he was nearer to Rome – still the key point. He temporarily came to terms with his northern rival Clodius Albinus, governor of Britain, recognizing him as a colleague. It is surprising that Albinus was deceived so easily. Septimius had time to march eastward and defeat his other opponent, Pescennius Niger, in a series of battles in Asia Minor and Syria. He was then in a position to renew hostilities against Albinus, who had advanced into Gaul and rallied the western provinces of the Empire in his favour. Perhaps Albinus also had been playing for time. The numbers engaged in the decisive battle near Lugdumum (Lyon) are reported as being equal, and the issue for long hung in the balance, but Septimius was completely victorious, deciding the battle by his use of cavalry as an independent arm.

Septimius Severus’ military ability was allied to shrewd political insight. On being proclaimed emperor, he had been quick to occupy Rome and disband the Praetorian Guard. He then re-established the Praetorians to suit his own convenience. In the past, the Praetorian cohorts had normally been recruited from Italy, but Septimius threw membership open to all legionaries. This meant in practice that Praetorians were picked from the Illyrian legions which had supported him. They continued to serve him admirably as an imperial corps d’élite in the course of his eastern campaign.

Having eliminated other imperial pretenders, Septimius undertook an effective punitive expedition against the Parthians, who had given support to Niger, his eastern rival. He also had to act promptly in Britain, for the province, stripped of troops by Albinus for his continental adventure, was badly exposed to Caledonian invaders from the north. But Septimius’ British campaign was incomplete and he was preparing to renew hostilities when he died at Eburacum (York) in AD 211.

Septimius Severus admired soldiers and believed in them, particularly in the soldiers of the Roman army. For him, their welfare was a paramount consideration, and one cannot help feeling that his attitude, despite its serious economic implications, was right. Roman civilization had come to depend completely on military power capable of defending the frontiers, and citizens who enjoyed the peace and comfort of metropolitan territories could at least be expected to support the defence effort with their tax contributions. Septimius, in fact, made sure that they did so.

Among other reforms which favoured the soldiers, he legislated that they should be able to marry legally while on service. This facility had not previously existed, though emperors in the past had given some sort of recognition to the relations which soldiers contracted with local women and to the children which resulted. Official attitudes on this subject seem to have been in conflict. On the one hand, the serving soldier was discouraged from forming local ties which might divert him from his principal allegiance to Rome. On the other, it was desired that he should feel at home in the army. The new legislation rectified anomalies. In any case Septimius’ son, colleague and successor, Marcus Aurelius Antoninus (known by the nickname of Caracalla) in subsequent years recognized the Roman citizenship of all freeborn provincials. The new constitutional enactment was not credited by an unimpressed posterity with generous motives, but regarded rather as a means of widening liability to tax. But it meant that civilians in general made a greater contribution to the defence budget. Of such a policy, Septimius would have approved.

■ Chaos and Recovery

In the middle decades of the third century AD, the Roman Empire appeared to be on the point of collapse. Barbarian attacks from without coincided with endemic discord within, and forces which were needed to resist invaders were exhausted in perennial struggles of disputed imperial succession. Between the years 238 and 253, emperors were elevated and replaced at the rate of something approaching one per year. Consuls in the old days had at least not been obliged to fight against other candidates for their yearly terms of office. The epoch has many characteristics of the Dark Age which was to descend on the western world five centuries later. Records are lacking and chronology is often hard to establish.

In the East, the situation had become particularly critical. Apart from other barbarian peoples, the Goths, who were already settled in what is now the Ukraine, carried out penetrating raids by land and sea into the interior of the Empire, plundering and destroying the cities not only of northern Asia Minor but of the Aegean coastline as far south as Ephesus. These invaders had discovered that the Roman Empire had a soft core. The frontiers were defended by troops and fortified, but once the crust of peripheral defences had been breached, an intruder had no problems. A world of unfortified cities and unarmed populations lay at his mercy.

Beyond the Euphrates, the situation had changed significantly. The Persians in their original homeland northeast of the Persian Gulf had, after the Seleucid period, survived as a Parthian vassal nation. But under the Sassanian dynasty (named from a dynastic ancestor), the Persians wrested hegemony from the Parthians and, at the beginning of the third century, became the new masters of the Parthian Empire. Nor were they slow to challenge Roman frontier positions east of the Euphrates. Very soon they captured key points and occupied territory which Rome had won from the Parthians in the course of their fiercely disputed campaigns against them.

In the year 242, a Roman financial administrator, Sabinus Aquila Timesitheus, became father-in-law of the young Emperor Gordian III and was appointed Prefect of the Praetorian Guard. In this position, he was the effective ruler of the Roman Empire, and his organizing ability was soon employed to produce excellent military results, especially on the eastern front, where Carrhae was regained from the Persians and a Roman puppet king re-established in Syria. But the death of Timesitheus brought an end to these successes. The Persian offensive was renewed. Shapur (Sapor), son of the first Sassanian king Ardashir (Artaxerxes), occupied Armenia and invaded Syria. The Roman Emperor Valerian (Publius Licinius Valerianus) conducted a campaign which ended in a disastrous attempt to relieve the Syrian city of Edessa in 260. The Emperor himself fell into Persian hands, nor is it recorded that he ever emerged from captivity. Christian writers, who attributed to him a sombre fate, were no doubt influenced by the fact that Valerian had been among the many emperors who persecuted the Christians. Christianity, however, was also persecuted by Shapur; for the new Persian Zoroastrianism was, unlike the old easy-going creed of the Achaemenids, a missionary religion, displaying all the intolerance which necessarily accompanies such a faith.

In the West, meanwhile, Gaul was overrun by the Franks and other barbarians, who penetrated into Spain and destroyed Tarraco (Tarragona). These invaders discovered, as the Goths had discovered in the East, that once the Roman frontier was breached, they need expect no further resistance. In 259, Marcus Cassianus Latinius Postumus, who was a pretender to the supreme imperial power and therefore formally at war with the Emperor Gallienus, Valerian’s son, revived the “Gallic Empire”, that notional establishment which had briefly struggled for existence in the days of Civilis. There was, of course, no question of Celtic nationalism, as under Vercingetorix. The independent régime simply claimed to defend Roman civilization in Gaul better than Rome could. Neither Gallienus nor his successor, Claudius, persisted in interfering with Postumus’ unquestionable service to the Roman cause. Regardless of the fact that he was usurping their authority, he demonstrated his power to repel the barbarians.

Thus relieved by his rival of responsibilities in Gaul, Claudius was able to concentrate his energies against Gothic enemies, who had now advanced westwards, threatening the Balkans and Italy. The Emperor’s victories freed the Mediterranean from the invaders and drove them back across the Danube, with the result that he went down to history as Claudius Gothicus. But the Goths were at this time stricken by plague, and Claudius was one of the many Romans who caught it from them. He died in 270, and his cavalry commander Aurelian (Lucius Domitius Aurelianus) was acclaimed as emperor by the army.

Aurelian, though successful against a multitude of Danubian enemies, was obliged to retract frontiers in this sector, as Gallienus and Postumus had been on the Rhine. The larger perimeter was no longer capable of defence. Aurelian reigned for five years, before losing his life as a result of a local conspiracy, and under his rule the Empire had time for recovery. Like the generals who had immediately preceded him in imperial power, he had realized that the barbarians could only be expelled if the earlier static concepts of frontier defence were abandoned. Germanic tribes (originally allied contingents settled within Rome’s frontiers) were ravaging Italy. Aurelian removed them after considerable vicissitudes by use of a highly mobile army, and he relied upon a cavalry force such as had already been developed by Gallienus. At the same time, the Emperor saw to it that Rome was provided, as a precaution against surprise, with strong new walls.

■ The Palmyrene Wars

Postumus in Gaul – attracting the allegiance both of Spanish and British provinces – had been an ostensible rebel, though in practice a very real ally against Rome’s barbarian enemies. In the East, by contrast, Odenatus, king of Palmyra, though loyally defending the Empire against Persia, had in fact created for himself a position of independence.

The Syrian desert city of Palmyra (as the Romans called Tadmor) had for long patrolled and policed the eastward caravan routes on which its prosperity depended. This was a natural preparation for military power. In other respects also, the Semitic, semi-Hellenized Palmyrene community was well qualified to fill the role of Roman sword-bearer in the East. The Persian army relied extensively on heavy cavalry protected by scale and plate armour. The Palmyrenes, as an antidote, deployed a combination of light and heavy cavalry, archers and, where requisite, sophisticated siege equipment. In addition, Palmyra had absorbed Roman administrative techniques, while the Persians, like their predecessors the Parthians, were organized on a local feudal basis without any central control such as might have given more permanent effect to their victories.

Odenatus, a prince of Arabian stock, whose dynasty had been established by Septimius Severus, had inflicted a crushing defeat on the Persians while they were still laden with plunder from their victory over Valerian. He had subsequently suppressed over-ambitious survivors of Valerian’s army, assumed command of surviving Roman troops in the area, and launched a counter-offensive in Mesopotamia, in the course of which he captured the Persian royal harem. For these exploits, he received due honour from Gallienus.

In the year AD 267, Odenatus’ beautiful and talented widow Zenobia inherited his power and ruled in the name of her young son; the king and his elder son by another wife had been assassinated in circumstances which are not fully clear. Zenobia showed herself independent of Gallienus, and Aurelian, though at first conciliatory, later felt obliged to assert his authority in the East. The Emperor’s officer had recovered Alexandria from the Palmyrenes, even before Aurelian marched through Asia Minor – where Palmyreme domination had been reluctantly endured. Zenobia’s general, Zabdas, wisely did not attempt to fight in mountainous country unsuited to cavalry tactics, but awaited the Roman legions in Syria. He was unable to defend Antioch, but made a second stand at Emessa (Homs). Here, the mailed Palmyren lancers drove Aurelian’s cavalry from the field, but Aurelian won the battle in their absence and dealt with them suitably when they returned in scattered units.

Only the desert, the Bedovin and the sun now defended Palmyra, whither Zenobia had fled. Not surprisingly, she made an appeal to Persia. But Aurelian bought off the Bedouin and fought off the Persians. while his army heroically maintained a siege. Zenobia, on a fast dromedary, tried to escape across the Euphrates by night, but she was overtaken and brought back to Aurelian a prisoner. Later, she graced his triumph in golden chains, but ended her days in peace at Rome, married to a senator.

When Palmyra fell, Aurelian put to death the advisers whom Zenobia had been glad to blame, but he spared the rest of the city. After his return to the Danube, however, Roman garrisons in the East were treacherously attacked, and in a second visitation he destroyed Palmyra utterly. He must have done so with some reluctance, for the city’s potential as a buffer state against Persia was considerable. But its annihilation now permitted Aurelian to impose himself on Gaul, which, after the death of Postumus, was ripe for the restitution of Roman authority.

■ Military and Civil Reorganization

The decade following Aurelian’s death was marked by another sequence of shortlived emperors. The year AD 284, however, saw the proclamation of the Emperor Diocletian (Gaius Aurelius Valerius Diocletianus) by troops in Asia Minor. Diocletian won the war against his rival and appointed Maximian (Marcus Aurelius Valerius Maximianus) as his colleague.

In 286, Diocletian permitted to Maximian the title “Augustus”, which indicated possession of the supreme power. From that time on, they ruled jointly, and in 293 each “Augustus” appointed himself a colleague who bore the title of “Caesar”. Four Imperial Headquarters, with their staffs, thus resulted. By regularizing procedures which had proved expedient in the past, Diocletian was in fact giving recognition to the inevitability of the collegiate principle. The Empire was too big for a single command. Troops might be transferred from Britain to the Danube in two months: perhaps less, if full use were made of Rhine river transport. But the Euphrates frontier was another matter. East and West were two Empires within a single civilization, and Diocletian wished to ensure that they should remain collegiate, not rival Empires. To some extent, their mutual independence was an accomplished fact which he was forced to recognise.

In re-establishing a co-optive procedure as the basis of imperial succession, Diocletian invoked another traditional expedient. Heredity notably in the family of Septimius Severus – based simply on blood-ties – had been productive of some grotesque results. Similarly, “praetorianism”, whether practised by the Guard itself or by the provincial legions, was simply an invitation to mutiny and murder. Because an emperor needed to be a soldier, it was too easily assumed that he needed to be nothing else. As in the first century AD, a blend of two principles was now expected to give best results. Co-option was confirmed by family affinities. The daughter of Diocletian and the step-daughter of Maximian married Galerius and Constantius, the two co-opted “Caesars”.

It was also arranged that the two “Augusti” should retire from office after 20 years and give place to their “Caesars”, who, assuming the supreme title, should appoint new “Caesars” as junior colleagues. Diocletian himself retired to his palace at Salonae (near modern Split in Jugoslavia). His choice of residence is itself significant. The imperial centre of gravity now lay in the Balkan peninsula and southeast Europe. Diocletian, like several of his imperial predecessors, had been of Balkan extraction. Rome was rapidly becoming no more than the ceremonial capital of empire. In practice, it was already merely a provincial capital, and the Senate was treated by Diocletian as if it were a body of town councillors. He never entered Rome during the first 20 years of his reign.

With his stern eye for realities and disregard for empty forms, Diocletian also relegated the old names of Republican magistracies to purely civil functions, and increasingly used distinct titles for military appointments. Like Septimius Severus, he realized that Rome’s greatest problem was one of recruiting, and he seems to have almost doubled the number of soldiers by increasing their pay. In order to do this, it was necessary to combat the monetary inflation which had long been associated with debasement of the Roman coinage. Diocletian went to the heart of the problem by exacting taxes in kind and maintaining his army with the proceeds.

Above all, Diocletian was an administrator and organizer, but it must not therefore be inferred that he was an “armchair” strategist. His reforms were worked out in the course of action and, like most Roman emperors who survived the first months of power, he had been obliged to fight for his position, suppress revolts and restrain barbarians. Maximian, his fellow “Augustus”, was an ambitious man, but he knew better than to challenge Diocletian on the field of battle.

Maximian, as Emperor of the West, had in fact his own military problems. Of these, the most intractable was presented by Carausius, a rebellious admiral of the British Channel fleet. Irrepressible, Carausius was for some time endured by the two “Augusti” as a kind of supernumerary colleague in Britain and north Gaul. Eventually, Maximian’s “Caesar”, Constantius, drove him from Boulogne and, continuing the war against Carausius’ murderer and successor, restored Britain to its former allegiance.

■ Constantine and Constantinople

Constantius died at York in AD 306, after a successful campaign against the barbarous Picts north of the Wall. His son Constantine was proclaimed “Augustus” by the British legions, but a period of complicated wars, negotiations, bandying of titles and dynastic marriages intervened before Constantine (known to history as “the Great”) attained the supremacy which Diocletian, for all his fourfold system of government, had really enjoyed. The fourfold system was, in fact, among the less durable of Diocletian’s institutions, and its obvious vulnerability to the old maladies saddened him before his death in 316.

Constantine developed the Roman army along lines which Diocletian had laid down and which had been apparent even in Aurelian’s time. Static frontier forces (limitanei) occupied forts in peripheral zones or manned the lines of river barriers. The best troops, however, were reserved as a mobile striking force (comitatenses) which could direct its energies as emergency required. The infantry units of this striking force were still termed legions, though their strength was reduced to about a third of the old Marian legion. It would seem, in fact, that the original legions had sometimes been split and apportioned between the frontier garrisons and the emperor’s mobile field armies. The mobile forces, of course, had more need of a strong cavalry element, but Rome had long been accustomed to rely on the barbarians who were settled in frontier areas to supply cavalry, such forces having being classed as numeri. There was a natural tendency, in the interests of security, to keep the barbarians on the frontier, away from the heart of the Empire, but in view of commitments to central mobility it could not indefinitely be upheld.

Constantine made one change which is symptomatic rather than important: he abolished the Praetorian Guard. The Praetorian cohorts were by now wholly redundant. Both their uses and abuses had been usurped by other sections of the army. The title of Praetorian Prefect was applied by Constantione to a purely civil official.

Constantine’s most monumental work was, of course, his building of Constantinople, the “New Rome” and second capital of the Empire. For this role the site of the ancient Greek city of Byzantium was chosen. A glance at the map will immediately make clear the economic and strategic importance of the position selected, at the centre of land communications between Europe and Asia and of sea communications between the Mediterranean and the Black Sea. Above all, Constantinople was ideally placed as a general headquarters for operations on the all-important Danube front.

Septimius Severus, with something less than his usual foresight. had destroyed Byzantium after the city supported his eastern rival Pescennius Niger. As a result, there remained no effective base or stronghold against the Goths, who in the following generation commandeered the fleets of Greek Pontic cities and swept down in their piratical raids into the Aegean Sea. In building the walls of his new capital, Constantine was affirming his faith in fortifications in general and in the importance of fortifying this particular point. Constantine’s fortifications are not those which now survive, but the position was eminently fortifiable. The barbarian invaders were never very successful in attacking fortified cities, and the walls of Constantinople withstood their attacks throughout many centuries to come.

Constantine is perhaps best known as being the first Christian emperor. In fact, he became a Christian on his death-bed, but before that date had, like other imperial pretenders of his generation, given support and encouragement to the Christians. The most immediate and tangible military effect of his attitude was the adoption of Christian battle standards. These featured a monogram compounded of the first two letters of Christ’s name in Greek (XP). Constantine also had the device painted on his soldiers’ shields, and it was first carried into battle when, in 312, he invaded Italy, to wrest power from Maxentius, son of Diocletian’s old colleague Maximian.

1 The form Caractacus rests on poor authority.