The underlying cause of the war was the fear by Sparta and her allies of the growing power of Athens. These two city states were the center of two power blocs whose influence was approximately equal. Athens controlled an empire comprising most of the states bordering the Aegean and Thracian seas as well as islands in the Aegean and off the Ionian coast. Her power principally rested on naval strength. Sparta and her allies, commonly called the Peloponnesian League, were the dominant land power of south and central Greece and had an influence extending westwards to Sicily through the fleet of Corinth.
Sparta and Athens signed the Thirty Years Treaty in 445. The immediate cause of the war was the claim by Sparta and her allies that Athens had breached the terms of the treaty. A series of precipitating incidents was commenced by Athens, who contracted an alliance with the Corinthian colony of Corcyra in west Greece. The Corinthians and their allies attacked Corcyra and Athens retaliated by commencing action against Megara, another Corinthian ally, and Potidaea, a Corinthian colony allied with Athens. The Corinthians protested to Sparta and ultimately the League decided that it would declare war if Athens did not withdraw her forces from Megara and Potidaea. Influenced by the statesman Pericles to reject Spartan demands the Athenians offered to discuss the matter but Thebes, an ally of Sparta, made an unprovoked attack on Plataea, an Athenian ally, in 431, and it finally pushed both powers into war.
First ten years
The early stages of the conflict were dominated by Pericles' insistence that Athens should avoid fighting the superior Spartan forces on land. Instead the citizens were kept within the 'long walls' that connected Athens with her port of Piraeus, even when the Spartans ravaged Attica. Pericles depended on the Athenian fleet to blockade Sparta, supply Athens and launch seaborne attacks on the enemy. His strategy, although sound, was interrupted by an outbreak of plague that raged in Athens and claimed Pericles himself as a victim. Although the Spartans made little use of this advantage the Athenians came under the influence of the extreme democrat Cleon, who advised the city to adopt the offensive. By 425 the Spartans had been forced to retreat but under the generalship of Brasidas they fought back, causing the Athenians to sue for peace in 421. This uneasy peace lasted until 415 and during this time, both parties engaged in diplomatic manoeuvrings and minor skirmishes occurred between their allies. In 416 Athens captured the neutral island of Melos and massacred its people; it also sent a war fleet to conquer Sicily, thus effectively reopening hostilities.
The Athenians suffered overwhelming defeats at Syracuse in Sicily, both on land and at sea. One of the Athenian leaders, Alcibiades, defected to Sparta and advised his new allies to blockade Athens by land. An alliance between Sparta and the King of Persia and the defection of several Ionic states from Athenian domination then threatened Athens' control of the sea. Meanwhile political upheavals in Athens resulted in a moderate government's coming to power and they achieved some strategic successes but a devastating naval defeat at Aegospotami by a Persian-backed Spartan fleet inflicted a final blow on Athens. In 404 the Spartans began to demolish the 'long walls' and Athens' period as a great power ended.