Pentecontaetia is the term used to refer to the period in Ancient Greek history between the defeat of the second Persian invasion of Greece at Plataea in 479 BC and the beginning of the Peloponnesian War in 431 BC. The term originated with a scholiast commenting on Thucydides, who used it in their description of the period. The Pentecontaetia was marked by the rise of Athens as the dominant state in the Greek world and by the rise of Athenian democracy, a period also known as Golden Age of Athens. Since Thucydides focused his account on these developments, the term is generally used when discussing developments in and involving Athens.

The Peloponnesian War was an ancient Greek war fought between Athens and Sparta and their respective allies for the hegemony of the Greek world. The war remained undecided for a long time, until the decisive intervention of the Persian Empire in support of Sparta. Led by Lysander, the Spartan fleet, built with Persian subsidies, finally defeated Athens and started a period of Spartan hegemony over Greece.