PELOPONNESIAN WAR, THE, was the great war waged towards the end of the 5th century B.C. by Sparta and the other members of the Peloponnesian Confederacy upon Athens and the Athenian empire. The cities of the Boeotian Confederacy under Theban leadership were Sparta's allies from the first. Syracuse and other Sicilian cities gave active help in the last part of the war. Argos, her hands tied by a treaty with Sparta, remained neutral during the first ten years, but, as a democracy, was benevolently inclined towards Athens. Persia at first held aloof, waiting her opportunity to reassert her dominion over the Greek cities of the Asiatic seaboard which Athens had liberated and added to her own empire. Athens, indubitably unpopular with many members of her empire, found small sympathy beyond its limits. Her maritime supremacy however held the malcontents firmly in check. As the war progressed, the whole Greek world became divided practically into two hostile groups, and both sides resented neutrality bitterly, as the cruel fate of Melos in B.C. showed. The war began on April 4, B.C., by a Theban attempt to surprise Plataea, Athens' ally and outpost on the northern base of Cithaeron. It ended on April 25, 404 B.C., when Athens capitulated. Thus it lasted 27 years, and Thucydides, writing its history after its close, definitely regards it as a single war, though a peace concluded between Athens and Sparta on April 11, 421 B.C., lasted technically some seven years. During these years there was heavy fighting in the Peloponnese in which Spartan and Athenian troops were engaged on opposite sides, and Thucydides' view is so far justified. The war divides into three main sections : (I) The "First" or "Archidamian" War, 431-421 B.C. ; (2) The "Sicilian" War, 421-413 B.C.; (3) The "Later" or "Ionian" or "Decelean" War, 412-404 B.C.
The true cause of the war was Sparta's fear of the growth of the power of Athens. This is Thucydides' own final judgment.
The whole history of the rise and power of the Athenian empire in the 5o preceding years justifies this view, though the immediate occasions of the war concerned Corinth, Sparta's chief naval ally, rather than Sparta. Since the peace of 445 B•C• Pericles had con solidated Athenian resources, made Athens' navy incomparable, concluded, in 433 B.C., a defensive alliance with the strong naval power Corcyra (Corinth's most bitter enemy), and renewed alliances with Rhegium and Leontini in the West. The very food supply of the Peloponnese from Sicily was endangered. In the Aegean, Athens could always enforce a monopoly of sea borne trade. To this extent the Peloponnesian War was a "Trade War," and on this ground chiefly Corinth appealed to Sparta to take up arms. The appeal was backed by Megara, well-nigh ruined by Pericles' economic boycott (he hoping thereby to com pel her once more to join Athens), and by Aegina, reluctant mem ber of the Athenian empire, heavily taxed, and claiming as a right established by treaty that Home Rule which Athens refused her. But had not Sparta herself been eager for war, peace would have lasted. She was but waiting the opportunity, which came when Athens was temporarily embarrassed by the revolt of her subject ally Potidaea in Chalcidice in the spring of 432 B.C. The rebel city held out until the winter of 43o B.C. and its blockade meant a constant drain upon Athenian military and naval resources. Sparta seized the chance. Confident of speedy victory, she re fused the offer of arbitration upon all disputed questions made her by Pericles, though the peace of 445 B.C. bound her to accept it. The ultimatum despatched to Athens was tantamount to the destruction of the prestige if not of the actual existence of the Athenian empire. At Pericles' urging the Athenian people stood firm and Sparta declared war. She had a bad conscience but a good war-cry, liberation of the Hellenes from Athenian despotism.