Of the life of Thucydides we know little beyond what he tells us. We believe he was born around 460–455, and we know that he was an elected general in 424. During that year he was in charge of forces that were unable to save Amphipolis from the Spartans. Democratic Athens frequently brought criminal charges against unsuccessful generals, so Thucydides went into a twenty-year exile during which he visited the Peloponnesus (including Sparta) and probably also traveled to Syracuse. Most of the time he probably spent in Thrace, where his family had longstanding connections. He returned to Athens after the war but apparently did not live to finish the History.

His education must have included some study with the sophists who were beginning to be popular in Athens during his youth. His style of writing, however, is uniquely his own, and he must be counted as one of the most original prose stylists of the Greek or any language.

The first phase of the Peloponnesian War began in 431 and lasted ten years. Although the Athenians met some reverses, they were victorious on the whole. Their greatest success in this period was the capture in 425 of a band of 120 Spartan citizen soldiers on a small island called Sphacteria, near Pylos. So much did the Spartans value their citizens that they were willing to sue for peace on virtually any terms to get their men back.