Justinian’s reign was the high-water mark for the Byzantine Empire. After his death in 565, the Eastern Roman Empire faced crisis after crisis. Constantly besieged by the Slavs and Bulgars in southern Europe, Persians in Mesopotamia, and from the mid-seventh century onward, Islam from Africa and the Levant, the Byzantine Empire found its military and fiscal resources relentlessly stretched. Still, despite defeats that deprived it of all its African and Asian possessions except Asia Minor, the Byzantine Empire maintained itself for centuries as a formidable eastern Mediterranean power.
Map 1.5. The Battle of Casilinum, 554. (a) Phase I: Narses forms his infantry, reinforced by dismounted heavy cavalrymen, into a three-rank formation flanked by mounted heavy cavalry. These cataphracts use the bow as their primary weapon rather than the usual lance or sword. The Franks form opposite in several unarticulated battle squares. They open the action by charging the Byzantine centre. (b) Phase II: The Franks’ ferocious attack successfully penetrates the first two ranks of Byzantine foot (1). As they engage the last line of infantry, Narses orders his cataphracts into action. The Byzantine horsemen press inward towards the enemy flanks (2). (c) Phase III: Fearing a charge by the Byzantine horsemen, the Frankish infantry press together into a large defensive square (1). Instead of charging, however, the cataphracts open a punishing fire into the flanks and rear of the tightly pressed Frankish square (2). Unable to reply to this attack without loosening their formation and opening themselves to a Byzantine heavy cavalry charge, the Franks slowly attempt to withdraw from the tightening noose (3). (d) Phase IV: The unarticulated Frankish formation loses cohesion as it retreats (1), an effect probably heightened by the presence of the Byzantine cataphracts on their flanks. Narses orders the cataphracts to exchange their bows for lances and swords, and the heavy cavalry charge into the flanks of the rapidly collapsing Frankish square (2). The ensuing slaughter results in the near annihilation of the Frankish force.
Unable to sustain a professional standing army, Emperor Constans II (r. 641–668) settled his mobile armies in specific districts called themes (from which the tactical unit thema gets its name). The new organization of themes introduced regionalized army groups under the command of a senior general or strategos. The themes now acted as both regional frontier troops and as mobile field armies. Outside of military organization, the themes were also an important part of imperial social structure. The soldiers of a particular theme were the legal holders of the land itself, a development that came in the form of imperial land grants within the particular region, similar to the land grants during the early Roman Empire. Although the soldiers did not work the fields or run farms on a full-time basis, their ownership brought about a personal stake in the defence of their respectivetheme. The theme became more than a military district; it was an economic and social entity as well.
Beginning in the mid-seventh century, soldiers of the themes supported themselves from land grants within their districts because the state’s only remaining means of supporting its soldiers seems to have been giving them land, most probably from imperial estates. For the most part, the themesacted as a defensive force, but later emperors would take an interest in regaining lost territory. Emperor Constantine V (r. 741–775) created several elite cavalry units called tagmata, first as a reaction to a rebellioustheme in north-west Anatolia, then for offensive campaigns against the Arabs and Bulgars.
The basic administrative and tactical unit of the Byzantine army from the seventh century was the bandum, consisting of about 400 soldiers commanded by a tribune, and later by a count. The banda were about equally divided into infantry and cavalry, with the dominant weapon system being heavy cavalry. Light infantry archers and javelineers usually accompanied the heavy infantry, acting as skirmishers and missile support. Five to eight banda (2,000–3,200 cavalry and infantry) formed a turma, two or threeturmae (4,000–9,600 soldiers) constituted a thema, and three or fourthemae together became a Byzantine field army, usually numbering 25,000 to 30,000 men. The entire Byzantine army was not very large, probably never exceeding 150,000 men in total. For all practical purposes, the thema replaced the legion as the premier strategic unit of manoeuvre in Byzantine warfare.
The Byzantine army differed from its Roman predecessor in one significant way – the level of professionalism in its military. Although the soldiers of the thema became increasingly a defensive militia force, the core of the Byzantine army were professional soldiers organized in homogenized cavalry or infantry units called tagmata, equal to the size of the thema. These soldiers were the best-trained troops in the empire, serving as Constantinople’s garrison and as the chief expeditionary force for the emperor. When the emperor went on campaign, the tagmata and local themaecombined to create a field army.
For the next 300 years Byzantine field armies enforced a conservative, careful strategy of limited military aims, seizing land in the Balkans from the Bulgars, and territories in Anatolia, Syria and Armenia from the Muslims. But by the turn of the millennium, the overall professionalism of the Byzantine army dropped precipitously, forcing emperors to rely increasingly on foreign mercenaries, especially Russo-Swedish soldiers called the Varangian Guard. The Byzantines were hiring small bodies of these mercenaries for expeditions as early as 911, with the Varangian Guard itself instituted by Emperor Basil II in 988. The Varangians lacked military lands and proved very loyal to Byzantine emperors who paid them well. By the beginning of the eleventh century, emperors added to their payrolls Normans, Germans and Turks, serving under their own officers in units with their own organization. As the themes declined in combat efficiency, these mercenaries began to replace the Byzantine army rather than merely complement it.