The Peloponnesian War Origins


Thucydides’ Explanation

After devoting the opening chapters of his history to arguing that the Peloponnesian War, fought between Sparta and Athens from 431 to 404, was greater than any previous war, Thucydides makes an often-quoted pronouncement (see box).

The Athenians and Peloponnesians began the war after breaking the Thirty Years’ Treaty which they had made after the capture of Euboea. As to why they broke the treaty, I have first written down the grievances [aitiai] and disputes [diaphorai], so that no one should ever have to enquire from what origin so great a war broke out among the Greeks. The truest reason [prophasis], but most concealed in word, I believe to be that the Athenians became powerful, filled the Spartans with fear, and forced them to go to war. But the following were the publicly mentioned grievances on each side, as a result of which they broke the treaty and embarked on the war. (Thucydides, I. 23. iv-vi)

He then gives a narrative of two episodes, concerning Corcyra and Potidaea, which served as grievances (I. 24–66); and of an assembly in Sparta at which Corinth and others urged the Spartans to act, other grievances, concerning Aegina and Megara, were mentioned, an Athenian delegation warned Sparta not to go to war lightly, and the Spartans did decide that Athens was in the wrong and they should go to war. Thucydides ends that section by repeating that Sparta was moved more by fear of Athens’ growing power than by the allies’ complaints (I. 67–88); and he then, to justify this view, gives his account of the growth of Athens’ power during the Pentecontaetia, the (nearly) fifty years since the Persian Wars (I. 89–118. ii). After that he resumes his narrative, with a congress of the Peloponnesian League, which approves Sparta’s decision to go to war; with an exchange of propaganda, in which Sparta begins with particular grievances but ends by demanding that Athens should restore their freedom to the Greeks; and with a speech by Pericles in Athens claiming that Athens was well prepared, and that if it were to give way on the grievances Sparta would come back with others (I. 118. iii-146).

Despite Thucydides’ hopes, subsequent generations have not accepted his account as definitive, but have been provoked to ask a variety of questions: in particular, what were his intentions in operating with two kinds of explanation? why, among the four grievances, did he single out two for detailed treatment and say so little about the others? what messages did he mean to convey about the responsibility of Athens, Sparta and Corinth for the war, and should we agree with him? It will be best to look in more detail at what he reports, and then to return to the broader questions.

The Grievances and Disputes

Corcyra (I. 24–55) was a colony of Corinth on an island off the north-west coast of Greece, and Epidamnus, on the mainland further north, was a joint colony of Corcyra and Corinth. When the democrats of Epidamnus expelled the oligarchs, and the oligarchs joined with the neighbouring Taulantians in attacking Epidamnus, the democrats appealed to Corcyra, but Corcyra (which, though comparatively democratic, had stronger links with the oligarchs) refused to help; and the democrats then appealed to Corinth. Fifth-century Corinth (cf. ill. 11) liked to maintain close and friendly links with its colonies, but had not succeeded with Corcyra, so it was glad to respond. Corinth sent fresh settlers to Epidamnus; Corcyra besieged the city; after Corcyra defeated the Corinthians in a naval battle off Leucimme, at the south end of Corcyra, Epidamnus capitulated to Corcyra. If we count back in Thucydides from the beginning of the Peloponnesian War, we obtain dates of 435 for Leucimme and the surrender of Epidamnus, and, supported by inscriptions, of 433 for what followed (but Diodorus narrates this affair under 439/8 and 436/5).

Each side devoted 434 to further preparations; in the spring of 433 a deputation from each went to Athens, and Thucydides gives a debate. The Corcyraeans claim that, as they were not listed as allies of either side in the Thirty Years’

Ill. 11 Corinth: temple of Apollo with Acrocorinth behind. © Steve Maehl /Shutterstock


Peace, they are entitled to join either side now; and that Athens, Corinth and Corcyra have the three best navies in Greece, and since war between Athens and the Peloponnesians is brewing it will be to Athens’ advantage to add Corcyra’s navy to its own. The Corinthians reply that Corcyra’s previous neutrality is a sign not of virtue but of wickedness, that they should be free to deal with Corcyra as they had supported leaving Athens free to deal with Samos (cf. p. 73 - but the cases were not parallel, since Samos was recognised in the Thirty Years’ Peace as a member of the Athenian bloc), and that the coming war is’ s till uncertain’. The Athenians devoted two days to the debate: on the first they listened to the speeches, and were inclined to favour Corinth, but on the second day they decided to make a purely defensive alliance with Corcyra. (Thucydides was probably present, and must have known more than he tells us: which Athenians changed their minds? and why? and what was Pericles’ position? Probably Pericles wanted to support Corcyra, as Plutarch claims [Per. 29. i], but Thucydides says nothing that would detract from his view of the Athenians’ unanimously following Pericles’ lead.) In making a purely defensive alliance the Athenians were hoping to keep their hands clean even if they were drawn into fighting against Corinth; and when they intervened in the battle (below) they insisted that they were not breaking the Thirty Years’ Peace.

Athens originally sent just ten ships, under three generals one of whom was Cimon’s son Lacedaemonius (the same generals are named in the first half of M&L 61 = IG i3 364 ~ Fornara 126, as drawing money for their expedition from the treasury of Athena): Plutarch regards his appointment as a move by Pericles to humiliate him, but more probably, if he had inherited his father’s opposition to Pericles (which is not certain), the appointment results from the strength of Pericles’ opponents in the assembly. In a battle at Sybota, between the south end of Corcyra and the mainland, the Corinthians were getting the upper hand and the Athenians had to intervene to prevent them from landing on the island. By then Athens had decided to send a further twenty ships (on the generals there is a disagreement between I. 51. iv and the second half of the inscription, and Thucydides is probably to be convicted of an error), and on their arrival the Corinthians withdrew.

Thucydides comments on the Athenians’ decision to support Corcyra that they really were expecting a war with the Peloponnesians (I. 44. ii). Probably in 434/3, so at the same time as or slightly before the alliance with Corcyra, by the decrees of Callias the Athenians put their finances in good order, paying outstanding debts to the sacred treasuries, combining a number of small treasuries in a treasury of the Other Gods, winding up the building programme on the acropolis (as a result of which the Propylaea was left unfinished), and resolving to spend further surpluses on the dockyards and walls (M&L 58 = IG i3 52 ~ Fornara 119: cf. p. 98). In 433/2 they renewed permanent alliances which they had made earlier with Rhegium and Leontini (M&L 63–4 = IG i3 53–4 ~ Fornara 124–5, where the original preambles have been replaced with preambles naming that year’s archon). It does indeed appear that as early as 433 the Athenians were envisaging a major war to which the west would be relevant.

The second grievance reported by Thucydides concerns Potidaea (I. 56–66), on the western prong of Chalcidice, a tribute-paying member of the Delian League but a colony of Corinth. He writes as if the Athenians decided to put pressure on Potidaea because of its Corinthian connections, but he goes on to show that they were also worried about the influence in the region of king Perdiccas of Macedon (whose attitude to Athens fluctuated but was currently hostile), and Potidaea’s tribute record suggests that the pressure had begun some years earlier. Athens ordered Potidaea to demolish part of its city wall, send hostages and stop receiving the annual magistrates from Corinth whom (remarkably) it was still receiving. Potidaea protested to Athens, in vain, and appealed to Sparta, which promised to invade Attica if Athens attacked Potidaea (but did not do so until the war proper began in 431). In 432 (but Diodorus’ date is 435/4), after paying their tribute in the spring, Potidaea and its neighbours revolted, many coming together in nearby Olynthus; Athens sent two expeditions, which went first to Macedon and then to Potidaea. This time it was Corinth which tried to keep its hands clean, sending not an official force but a body of volunteers and mercenaries (cf. ‘privately’ in I. 66). There was a battle, in which the Athenians were victorious; they settled down to an expensive siege, which lasted until Potidaea capitulated in 430/29 (cf. p. 117).

Next Thucydides reports a Spartan assembly (I. 67–88), at which Corinth and other members of the Peloponnesian League urged Sparta to take action. Here two other grievances were mentioned. Aegina, in the Saronic Gulf, complained that Athens was not allowing it the autonomy promised in ‘the treaty’ (I. 67. ii, cf. 139. i, 140. iii). Thucydides gives no indication of the basis for the claim, not even whether ‘the treaty’ was the Thirty Years’ Peace or a separate treaty between Athens and Aegina (IG i3 38 is too small a fragment to be helpful; in 432 Aegina paid less than half of the 30 talents tribute which it had paid at least until 440); but in 431 Athens was to expel the inhabitants of Aegina, alleging that they were ‘not least responsible for the war’ (II. 27. i: cf. p. 114).

The Megarians, on the Isthmus of Corinth, complained that they were being excluded from the harbours of the Athenian empire and ‘the Attic agora’ because of a dispute over sacred land in the frontier region near Eleusis and the harbouring of runaway slaves (I. 67. iv, cf. 139. i-ii, 140. iii-iv, 144. ii). Plutarch (Per. 29. vii-31. i) shows that this was one of a series of measures, the sequence of which was probably: by a decree of Pericles Athens sent a herald with a moderate statement of Athens’ case; then came the exclusion decree; after the Megarians killed a herald called Anthemocritus, a decree of Charinus (which must be dated 431: cf. p. 113) committed Athens to implacable enmity and invasions of the Megarid twice a year. Pericles refused to weaken over Megara (when he insisted that the text of the exclusion decree could not be taken down, an opponent suggested that it should be turned to face the wall). On both occasions when Aristophanes alludes to the causes of the war, he focuses on Megara, with stories (different, and probably both invented: the chorus responds to the second, ‘I never heard that before’) of Pericles’ involvement for disreputable personal reasons (Acharn. 514–38, Peace 605–18; cf. Cratinus, fr. 38. 44–8 Kassel & Austin) - which were taken seriously by later writers. Thucydides, though he tells us little, indicates that this was the grievance particularly stressed by the Spartans.

Despite an ingenious attempt by de Ste. Croix to argue otherwise, the exclusion decree must be seen as an attempt to prevent Megara from trading with Athens and the empire: the Athenians realised that their control of the sea could be exploited to their own and their friends’ advantage and their enemies’ disadvantage (cf. pp. 187–8), and Aristophanes in Acharnians (535, cf. 719–835) represents the Megarians as starving because of the decree. Megara since the Thirty Years’ Peace had been a member of the Peloponnesian League: was Athens breaking the peace? There were no international lawyers, and no standing body which could decide hard cases; the history of the late fifth century shows that a treaty was broken if people chose to regard it as broken, or not if they chose not. The likelihood is that when the peace was made economic sanctions had not been envisaged and so were not explicitly forbidden, and therefore Athens was not breaking the letter of the peace; but it may well have seemed that Athens was overreacting to a small provocation. (Megara had supported Corinth in 435 and 433, but we do not know how long this feud had been going on, and both the first decree and the second may have been earlier than the Corcyraean episode.)

Sparta and Athens

In Thucydides’ account of the Spartan assembly a Corinthian speech stresses the Athenians’ ambition and contrasts their energy with Sparta’s slowness. An Athenian deputation is given permission to speak, refuses to respond to the grievances, emphasises Athens’ strength and urges Sparta to think twice before going to war; the empire is justified as the natural exercise of power (I. 72 with 73–81: on this speech cf. p. 189). Of the Spartans, king Archidamus recommends a gradual approach but the ephor Sthenelai’das in a laconic speech insists that Athens is in the wrong so war is necessary - and he gained a very public vote of approval by calling on the Spartans not merely to shout but to stand in different places to declare their opinions.

The Delphic oracle was consulted and gave Sparta its support (I. 118. iii). During the war the Athenians neither stayed away from Delphi nor were debarred from visiting it, but access to and the status of Delphi was the first matter to be mentioned in the truce between Athens and Sparta in 423 and in the Peace of Nicias in 421 (Thuc. IV. 118. i-iii, V. 18. ii).

To commit the Peloponnesian League to war, Sparta had to obtain a majority vote from the members. A congress was held (1.119-125. i), to which Thucydides assigns a second Corinthian speech, and the members did vote for war.

By now it was fairly late in 432. For the winter of 432/1 Thucydides reports an exchange of propaganda (I. 125. ii—146). The Spartans called on the Athenians to expel those tainted by the curse on the Alcmaeonid family (resulting from the killing of Cylon’s supporters when he tried to make himself tyrant, in the seventh century), an attempt to undermine the position of Pericles, whose mother was an Alcmaeonid. Athens responded with Spartan curses, resulting from the death of Pausanias in the 460’s (cf. p. 31), perhaps aimed at Pericles’ xenos Archidamus. Sparta then called for an end to the on-going grievances (the siege of Potidaea, the status of Aegina, and particularly the sanctions against Megara); and finally (in effect invoking Thucydides’ ‘truest reason’) announced that Sparta wanted peace, and there could be peace if Athens would leave the Greeks autonomous (for this theme cf. Thucydides’ comment on support for Sparta at the outbreak of the war, II. 8. iv). Thucydides ends book I with the first of the speeches attributed to Pericles: Sparta is refusing Athens’ offer to go to arbitration; the grievances are mere pretexts, and if Athens gave way on them Sparta would come back with others; Athens is better prepared than the Peloponnesians, but must not throw away its advantages by fighting in Attica or trying to enlarge the empire; it should reply firmly. As G. W. Hunt put it, in the context of a British warning to Russia in the 1870’s ,

We don’t want to fight, but by jingo! if we do,
We’ve got the ships, we’ve got the men, we’ve got the money too.

Athens did reply firmly.

In the spring of 431 the Thebans (allies of Sparta) made a sudden attack on Plataea (geographically in Boeotia, on the route fromThebes to the Peloponnese, but an ally of Athens for nearly a hundred years): the attack misfired and, perhaps in breach of an agreement, the Plataeans killed their captives (Thuc. II. 2–6). This attack allowed the Athenians to claim that the Peloponnesians were in breach of the peace. And so the war began.

Whose Fault?

Formally it was the Peloponnesians who judged that Athens had broken the peace, and who declared war, and in spite of their initial confidence the Spartans were later to feel guilty about this (Thuc. VII. 18. ii); but Thucydides has referred to the grievances of each side against the other (I. 23. vi, 146). Some scholars have thought that he originally concentrated on the grievances and only later came to see the importance of his truest reason, and that book I as we have it has been rewritten in the light of that change; but the book’s organisation, though complex, is coherent, and the truest reason is so widespread in book I that it is hard to imagine an earlier version which lacked it. More probably Thucydides was from the beginning operating with two levels of explanation. He was a historian proud of his ability to do better than others, as he made clear in ch. 20, towards the end of the opening section of book I; and in his account of the causes of the war he both emphasised the ‘truest reason’, which was ‘most concealed in word’, and gave his own version of the ‘publicly mentioned grievances’.

His choice of words for cause, ‘grievances and disputes’ contrasted with ‘reason’ (prophasis: a person’s or a state’s reason for acting), need not trouble us: the words are appropriate in their context, but he himself calls the grievances a reason in I. 118. i and 146. What is more important is that the grievances were publicly mentioned while the reason was concealed, and that the reason was truest. The reason is certainly not concealed in book I, from the expectation of war when Athens agrees to support Corcyra to Sparta’s final demand. Presumably, when the war had started, other people focused on one or more of the grievances - Aristophanes perhaps reflected public opinion in Athens in blaming Megara and Pericles’ obstinacy, the Corinthians perhaps blamed the volunteers who had gone to fight for Potidaea - and Thucydides is showing that he knows better. There are various possible explanations for his giving detailed accounts of two grievances but not of the other two. If Aristophanes is true to Athenian public opinion, Thucydides is perhaps reacting against that; and if the suggestion that Pericles had disreputable personal reasons for not giving way over Megara was widespread, it would not suit Thucydides the admirer of Pericles to dwell on that. Thucydides was also a patriotic Athenian: in his detailed accounts Athens makes an alliance with Corcyra which it is entitled to make, and limits its support so that the conflict with Corinth does not escalate; and it is within its rights in coercing Potidaea as a member of the Delian League: the suspicious reader may wonder if Athens’ treatment of Aegina and Megara was harder to justify.

His narrative shows Athens technically in the right over the grievances, and willing to go to arbitration when Sparta was not (but it is easy to score points by offering arbitration when it is unlikely that acceptable arbitrators can be found), and shows a slow Sparta pushed towards war by Corinth (Corinth was indeed the strongest member of the Peloponnesian League after Sparta, but would arguably have seemed less prominent if Thucydides had devoted equal space to all grievances). He traces the growth of Athenian power from the beginning of the Delian League, innocent in intention and accepted by Sparta (but after the Thirty Years’ Peace, which tried to establish a balance between Athens and Sparta, he mentions only Athens’ war against Samos, whereas to justify his view of the truest reason he should - and could: cf. pp. 74–5 - have done more to show why that treaty did not establish a sustainable balance). Although he formulates his truest reason to state that fear of Athenian power forced Sparta to go to war, the verdict which he has implied to most modern readers is that the Peloponnesians were in the wrong in making war on Athens.

We should not make too much of the Corinthian pressure: it suited Thucydides to contrast Athens’ energy with Sparta’s slowness, and the Corinthians could actually have made that contrast. However, the view fashionable in the early twentieth century that Athens was in competition with Corinth for control of trade with the west was based on an anachronistic view of trade and the state’s interest in it. The willingness of Sparta, or at any rate some Spartans, to support Samos in 440, and the attitude of Sthenelai’das in 432, warn us not to see too much reluctance in Sparta: nearly all those who begin a war like to believe, and to convince others, that they are in the right, and it was the grievances which enabled Sparta to do that. As for Athens … my judgment would be that at any rate the Athenians did not try very hard to avoid war. When they could have stayed out of the conflict between Corinth and Corcyra (cf. Thuc. I. 40. iv), allowing them to weaken each other (cf. 44. ii), they chose to make an alliance which could easily and in fact did lead to their fighting against the Corinthians, and at the same time they started preparing for a major war. Potidaea might have been put under pressure in 432 even if there had not been a Corinthian interest there, but what Athens did was bound to annoy Corinth; we know too little about how far back the grievances of Aegina and Megara reached (but the First Peloponnesian War had demonstrated the advantage to Athens of having Megara on its side: cf. pp. 47–50). The Athenian speech in Sparta was not calculated to turn away wrath; the offer of arbitration implies confidence that Athens was in the right and/or that the offer would not be taken up; the message of Pericles’ speech was that appeasement would not work, Athens was better prepared than the Peloponnesians, so if the war must come let it come.

I believe that the Athenians, and Pericles in particular, realised that Sparta could not tolerate their continuing and growing power but sooner or later they would have to fight for it; they were certainly not prepared to avoid war by giving up their ambitions; they adopted a high-risk strategy in the hope - which was fulfilled - that the inevitable war would come in circumstances in which they were better prepared than their enemies and could claim to be in the right. Thucydides’ judgment about the truest reason is to be accepted, but I should go further than he did on Athens’ willingness to provoke war. It was indeed to be a war about the power of Athens.


See in general de Ste. Croix, The Origins of the Peloponnesian War, Kagan, The Outbreak of the Peloponnesian War, also Cawkwell, Thucydides and the Peloponnesian War, ch. 2; Lazenby, The Peloponnesian War, ch. 2; Pelling, Literary Texts and the Greek Historian, chs. 5, 8. The view that a difference can be detected between Thucydides’ early and his later view of the causes was supported by A. Andrewes, ‘Thucydides and the Causes of the War’, CQ2 ix 1959, 223–39; rejected by D. Whitehead, ‘Thucydides: Fact-Grubber or Philosopher’, G&R2 xxvii 1980, 158–65, Rhodes, ‘Thucydides on the Causes of the Peloponnesian War’, Hermes cxv 1987, 154–65.

On the words aitia (‘grievance’) andprophasis (‘reason’) see particularly de Ste. Croix, The Origins of the Peloponnesian War, ch. 2; L. Pearson, ‘Prophasis: A Clarification’, TAPA ciii 1972, 381–94 = his Selected Papers, 120–33.

On the Athenian Decrees proposed by Callias (M&L 58 = IG i3 52 ~ Fornara 119), it has been shown that dating the two decrees to the same day depended on over-bold restoration; but they may still belong to the same year, and, though later dates have been proposed (see, e.g., Samons, Empire of the Owl, 113–38), I still believe that the year is 434/3, i.e. before the series of loans to the state recorded in M&L 72 = IG i3 369 (cf. pp. 97–8). It is possible that the proposer of the second decree was not Callias (C. W Fornara, forthcoming). J. R. Grant, ‘A Note on the Tone of Greek Diplomacy’, CQj xv 1965, 261–6, argued that the Athenian speech at Sparta in 432 was not as provocative as modern readers are inclined to think.

On Athens’ dispute with Megara, de Ste. Croix, The Origins of the Peloponnesian War, ch. 7, advanced an interpretation of the exclusion which has not found acceptance, but I do accept his chronology of the episodes mentioned by Plutarch; for alternative chronologies see C. W Fornara, ‘Plutarch and the Megarian Decree’, YCS xxiv 1975, 213–28; Cawkwell, Thucydides and the Peloponnesian War, 111–14. P. A. Brunt, ‘The Megarian Decree’, AJP lxxii 1951, 269–82 = his Studies in Greek History and Thought, ch. 1, stresses that the exclusion decree need not have been very recent when the Megarians complained about it in 432.

The theory of a war to control trade with the west was advanced by Cornford, Thucydides Mythistoricus, 1–76, and Grundy, Thucydides and the History of His Age, ch. 15; it was answered at the time by G. Dickins, ‘The True Cause of the Peloponnesian War’, CQ v 1911, 238–48; and for a more recent rebuttal see de Ste. Croix, The Origins of the Peloponnesian War, ch. 6, esp. 214–20.

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