The Cause of the Peloponnesian War
The immediate cause of the Peloponnesian War was Corinthian opportunism. Thucydides is mistaken in his famous assertion that "[w]hat made war inevitable was the growth of Athenian power and the fear which this caused in Sparta". Both powers had demonstrated a reluctance for head-on war over matters peripheral to their respective spheres of influence. On the eve of the war, Sparta had to be compelled into open warfare by Corinth over matters not really vital to Corinth, much less vital to Sparta. On the other side, Pericles —of all people the one most likely to push his fellow Athenians toward war— would have been willing to submit to arbitration had not the slow-moving but deliberate Spartans backed him into a corner with their ultimatum. What was and was not inevitable about Athenian expansion will be discussed below. In the end what made the war inevitable was that Sparta had either to go to war with Athens or to give up its leadership of the Peloponnesian League —a leadership that it had enjoyed for well over a hundred years and that was integral to its very survival. It was Corinth that forced upon Sparta this Hobson's choice.
While the shift in Athens' stature from naval to overall leadership against Persia was not an inevitable development, the original Athenian naval leadership against Persia was all but inevitable given any number of factors. As the largest entrepot in Hellas, Athens had its largest navy. Their already superior navy was enormously enhanced after the discovery in 483 of silver at Laurium. In the words of Herodotus, "Themistocles' judgment proved the best at an important moment"; he persuaded Athenians to retain the mines' profits in the public coffers in order to build 200 ships. Of course, Athenians had a greater interest than most Hellenes in the direction from which Persia threatened. In regard to trade and to metropolis-colony relationships, Athens was the city-state most involved in the Aegean and Ionia, in contradistinction to, say, Corinth, which was more oriented to the west in such business. Due to such factors, it was "inevitable" that Athens should assume the leadership of the Delian League when Sparta gave it up because their own leaders, notably Pausanias, proved recalcitrant when abroad and because "Sparta's ongoing need to keep its army at home most of the time to guard against helot revolts also made prolonged overseas operations difficult to maintain".
So far we can agree with Thucydides if we give a charitable reading to his assessment of the inevitability of events. We can even go further and say that the subtle change from Delian League to Athenian Empire was brought about by the ostensibly unavoidable conscription of poleis into (or the forcible maintenance in) the alliance —a necessity in the face of the Persian menace. However, when, in the second quarter of the fifth century, the real Persian threat virtually disappeared, Athens chose to pursue a policy of true self-interested imperialism. This choice was not inevitable, nor was it inevitable that war should follow. Thucydides makes both these points clear in the following passage, in which private Athenians are defending Athens against the charges brought by Corinth before the Spartan assembly (italics added):
We did not gain this empire by force. It came to us at a time when you were unwilling to fight on to the end against the Persians. At this time our allies came to us of their own accord and begged us to lead them. It was the actual course of events which first compelled us to increase our power to its present extent: fear of Persia was our chief motive, though afterwards we thought, too, of our own honour and our own interest.... We have done nothing extraordinary, nothing contrary to human nature in accepting an empire when it was offered to us and then in refusing to give it up. Three very powerful motives prevent us from doing so —security, honour, and self-interest. And we were not the first to act in this way. Far from it. It has always been a rule that the weak should be subject to the strong; and besides, we consider that we are worthy of our power. Up till the present moment you, too, used to think that we were; but now, after calculating your own interest, you are beginning to talk in terms of right and wrong.... we urge you, now, while we are both still free to make sensible decisions, do not break the peace, do not go back upon your oaths; instead let us settle our differences by arbitration, as is laid down in the treaty. If you will not do so, we shall have as our witnesses the gods who heard our oaths. You will have begun the war....
The whole of the passage from which this is taken, which comprises Book 1.75 through 78, clearly illustrates two points in direct contradiction to Thucydides' famous etiology: First, the nature of Athenian imperialism was, by the middle of the century, a matter of choice, not inevitability. Secondly and most importantly, the Peloponnesian acceptance of the Thirty Years' Peace was an explicit acceptance of the nature of Athenian imperialism. Only direct military confrontation could break the treaty, and Athens was innocent of such confrontation. Therefore, Athens cannot be said to have caused the Peloponnesian War. It may be countered that Athenian policies would likely have started some war eventually, but that would be conjecture. We are here confined to facts and to this war.
The Megarian Decree did not cause the war. The claim of the Spartans' second embassy to Athens "that war could be avoided if Athens would revoke the Megarian decree" was so much smoke. Like the demand of the first Spartan embassy that "the Athenians should 'drive out the curse of goddess'", it was meant to establish "a good pretext of making war". These demands had several other purposes: First, these demands, the Spartans hoped, would placate the gods. As Martin puts it, 
The Spartan refusal to honor an obligation imposed by an oath amounted to sacrilege. Although the Spartans continued to argue that the Athenians were at fault by refusing all concessions, they nevertheless felt uneasy about the possibility that the gods might punish them for refusing their sworn obligation.
Secondly, both these demands were attempts to diminish the stature of hawkish Pericles. Everyone knew that the "curse of the goddess" was brought about by, among others, an ancestor of Pericles; also, the Megarian Decree was Pericles' work. Thirdly, these demands presented purported reasons for war that diverted attention from the real reason: Corinthian aggression.
That the Megarian Decree was not a decisive issue can be seen in Pericles' reference to it —in regard to war— as a "trifle". Donald Kagan tells us that "[w]e may be sure... that the Megarian Decree was not a technical breach of the peace". Athens had every right to regulate the economic affairs of its empire. F. A. Lepper, as quoted by Kagan, gives us a good idea of what the decree was about; he states that it "may have been a late step in a gradual 'cold war' that Athens had been waging against Megara for some years". The only line in Thucydides that suggests that the decree was "contrary to the terms of the treaty" is offered by the Megarians themselves. Much later, Pericles more accurately states that "in the treaty there is no clause forbidding... our decree against Megara". Moreover, in the words of G. B. Grundy, "Neither Thucydides nor Aristophanes, nor any other ancient writer, gives any real clue to the reasons which induced the Peloponnesian League generally to attach such importance to the Megarian decree". Hence it is safe to say that the Megarian Decree, whatever it may have meant as a rallying point, did not cause the war.
The crisis at Epidamnus was brought into being by unilateral Corinthian initiative; it was not the result of any general Peloponnesian defensive necessity. In regard to Corinthian ambitions there, J. K. Davies writes,
The fact that the Spartans suspected 'private interests' sounds sour but makes sense both in the 450s and the 430s, in that any extension [sic] Athenian influence outside the Aegean into Central Greece or towards the West affected Corinthian interests very directly but Spartan interests hardly at all.
Thus Corinth represented strictly its own interests in regard to Corcyra. Even at that, Corinth was reaching; Epidamnus was too far off the beaten track to be of any real consequence to Corinth, and Illyria had no special inducements. On the other hand, Corcyra, with Greece's second-largest navy, did pose a potential impediment to Corinthian expansion in the north and west. Thucydides tells us that "it was a fact that Corcyra lay very conveniently on the coastal route to Italy and Sicily". Besides having been the mother city of the navally powerful Corcyra, Corinth had long before been, according to Martin, "the foremost ship-building center of Archaic Greece". Thus it was doubly insulting to Corinth's pride that its colony —though subsequently entirely independent— should both prevent its interference in Epidamnus and also snub Corinth in the traditional religious festivals that harkened back to the metropolis-colony relationship that they had shared. More to the point, in the words of Kagan,
Corinth, with a proud history as a commercial, industrial, artistic, and naval power, had seen her prestige shrink in comparison with the superpowers who had arisen since the middle of the sixth century. ...she determined to build a sphere of influence in the northwest of Greece to compensate for her diminished prestige elsewhere.
That Corinth's intrusion in Epidamnus was a case of adventurous expansion and not defense can be seen in the facts that Epidamnus was too far north to pose any threat to established Corinthian interests and that Corcyra itself had not, at any rate, interfered with Corinth or its trade.
It may be argued that Epidamnus was, in relation to the Thirty Years' Peace, an open city and therefore that Corinth had every right to send colonists and forces there. If so, then Corcyra, metropolis to Epidamnus, certainly had no less right. Be that as it may, the Corcyraeans offered to submit the quagmire to arbitration. The Corinthian response was to enlarge the contest by bringing in a good number of allies. Corcyra, which "had no allies in Hellas", was left with no choice but to give up power in its own neighborhood or to seek help from the only possible source, Athens. That Athens was free wihin the Thirty Years' Peace to ally with Corcyra is pointed out by both Corcyra and Corinth in their arguments at Athens. It was duplicitous of Corinth to claim that it was wrong for Corcyra and Athens to join in a legal and defensive alliance at the same time that Corinth was enlisting help from all over Hellas in the fight against Corcyra.
That Corinth was the aggressor in the actual military confrontation is plain; Corinth took the battle to Corcyraean waters. Dramatically to underscore the strictly defensive (and therefore unquestionably legal within the Thirty Years' Peace) nature of Athenian participation, the Athenian triremes "did not openly join the battle" until Corcyraean defeat became probable. Therefore, if there was any contravention of the Thirty Years' Peace treaty at Sybota, the culprit was Corinth.
Culpability for the Potidaean conflict is even more unambiguous —as is reflected in the amount of attention Thucydides gives Potidaea relative to that given Epidamnus. In Warner's translation, Thucydides gives the conflict over Epidamnus 702 lines of text, the report on Potidaea 177 lines. Thus despite the greater complexity of the affair at Potidaea (involving as it did the foreigner Perdiccas' strategic machinations) and its greater magnitude (involving Macedonian interference with several cities subject to Athens), Thucydides gives it far less reportage. The reason for this difference is clear: There is no question about responsibility. He tells us right off the top that "Corinth was searching for means of retaliation". In terms of upsetting the treaty of 446/445, it is an open and shut case. As a "signatory" of that peace accord, Corinth swore an oath to honor Athens' hegemony over Potidaea despite the metropolis-colony relationship involved. To send forces to Potidaea under any circumstances was a clear breach of the treaty.
Corinth presented Sparta with a fait accompli; the Thirty Years' Peace had been broken. Contingent as Sparta's existence was on its leadership of the Peloponnesus because of the potential instability of its helot-based infrastructure, Sparta had no choice but war. Corinth was threatening "to join a different alliance". Sparta's only "alternative" was to sink into oblivion. Corinth had caused what we now call the Peloponnesian War.
There has never been absolute peace anywhere. Therefore, historians have always had difficulty in defining wars and dating their beginnings. Never in the history of historiography have scholars given the definition and dates of any other war so much attention as has been given those of the Peloponnesian War. If we define the Peloponnesian War as that which began in 432/1 between the Peloponnesian League and the Athenian Empire, and if we accept the preponderance of the evidence available to us, then its etiology is manifest: The cause of the war was Corinthian aggression.
 Thucydides, History of the Peloponnesian War, trans.: Rex Warner (London: Penguin, 1972), Book 1.23.
 Thomas R. Martin, Ancient Greece (New Haven: Yale University Press, 1996), pp. 96, 104. Other historians appear to be uncertain of the date.
 Herodotus, The History, trans.: David Grene (Chicago: University of Chicago Press, 1987), Book 7.144.
 I underscore the importance of naval force because Thucydides does so: "Hellenic navies... brought in revenue and they were the foundation of empire." (Book 1.15)
 Martin, p. 106.
 There is no point here to introducing our very modern inhibitions against conscription.
 I am reproducing so large a piece of Thucydides because of its importance to my argument.
 It will be argued that Corinth, not Athens, was responsible for the military confrontations over Epidamnus and Potidaea
 Thucydides, Book 1.139.
 Thucydides, Book 1.126.
 Thucydides, Book 1.126.
 Martin, p. 152.
 Thucydides, Book 1.140.
 Donald Kagan, The Outbreak of the Peloponnesian War (Ithaca: Cornell University Press, 1969), p. 267.
 Kagan, p. 255.
 Thucydides, Book 1.67.
 Thucydides, Book 1.144.
 G. B. Grundy, Thucydides and the History of his Age (London: John Murray, 1911), p. 77.
 J. K. Davies, Democracy and Classical Greece (Cambridge, US: Harvard University Press, 1993), pp. 65-66.
 Thucydides, Book 1.44.
 Martin, p. 59.
 Martin p. 221.
 Thucydides, Book 1.31.
 Thucydides, Book 1.35 and 40.
 Thucydides, Book 1.49.
 I am addressing responsibility for the breaking of the Greek treaty, not Macedonian interloping. Perdiccas cannot be faulted for breaking a treaty to which he was not a party.
 Thucydides, Book 1.56.
 Thucydides, Book 1.71.