In 432, Sparta and its allies, fearing the growth of Athens's power and its aggressive policies, decided to go to war. In subsequent negotiations, the Spartans raised specific demands that, if accepted, might have avoided war. Urged by Pericles, the Athenians decided not to negotiate about ultimatums; any disagreements were to be resolved by arbitration, as envisaged by the Thirty Years' Peace of 446. A few months later, fighting began. After almost three decades of war, interrupted by a short period of ineffective peace, Sparta prevailed. In 404, starved and exhausted, the Athenians capitulated.

Pericles died in 429. Only the outbreak and first two years of this war were his responsibility. Thucydides explains its catastrophic outcome by the mistakes and rivalries of Pericles' successors. In his view, Pericles' plan, strategy, and calculations were sound; had the Athenians followed his advice, they could have won (2.65.8-10). He supports this judgment by a comprehensive assessment that he lets Pericles formulate in a speech before the war (1.140-44): Financial reserves and a centralized, unified process of deliberation and decision-making are crucial for success in war. Both are typical of Athens but not of the enemy. In a single land battle, the Peloponnesians can take on the rest of Greece, but they do not have the resources to fight the type of war that is required to defeat the Athenians. They are farmers, not sailors, used to engaging in short infantry wars with neighbors but unable to spend much time away from home and lacking experience in fighting long wars overseas.