Why did Athens and Sparta go to war, and how much should we trust Thucydides?

Athens and Sparta went to war, Thucydides said, because Athens had grown so powerful and Sparta was alarmed. His summary: "The growth of the power of Athens, and the alarm which this inspired in Lacedaemon, made war inevitable". The Athenian empire had grown out of the Delian League, whose allies had been reduced to tribute-paying subject states. That tribute funded a powerful fleet and massive public works in Athens, causing resentment. The war began in 431 BC when the Spartan king Archidamus II invaded Attica.

It went badly for Athens in ways nobody planned. Pericles advised avoiding open battle with the far more numerous Spartan hoplites and relying on the fleet. Then, in 430 BC, a plague hit Athens, ravaging the densely packed city and becoming a significant cause of its final defeat. In Sicily, the entire Athenian fleet was destroyed and virtually the entire army sold into slavery. In 405 BC Sparta won the decisive battle of Aegospotami, and Athens fell the following year.

Behind all this stands a historian. Thucydides, an Athenian who fought early in the war, was exiled in 423 BC and wrote his account while living in the Peloponnese. Scholars regard him as reliable and neutral between the two sides, with one caveat: he admits his speeches are his interpretation of the general arguments, not accurate records. He wanted his work to be "a possession for all time", and later readers have taken that at its word.