Pentecontaetia (Greek: πεντηκονταετία, "the period of fifty years") 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 433 BC. The term originated with Thucydides, who used it in his 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. Since Thucydides focused his account on these developments, the term is generally used when discussing developments in and involving Athens.
Shortly after the Greek victory of 479 BC, Athens assumed the leadership of the Delian League, a coalition of states that wished to continue the war against Persia. This league experienced a number of successes and was soon established as the dominant military force of the Aegean. Athenian control over the league grew as some "allies" were reduced to the status of tribute-paying subjects and by the middle of the 5th century BC (the league treasury was moved from Delos to Athens in 454 BC) the league had been transformed into an Athenian empire. Athens benefited greatly from this tribute, undergoing a cultural renaissance and undertaking massive public building projects, including the Parthenon; Athenian democracy, meanwhile, developed into what is today called radical or Periclean democracy, in which the popular assembly of the citizens and the large, citizen juries exercised near-complete control over the state.
The later years of the Pentecontaetia were marked by increasing conflict between Athens and the traditional land powers of Greece, led by Sparta. Between 460 BC and 445 BC, Athens fought a shifting coalition of mainland powers in what is now known as the First Peloponnesian War. During the course of this conflict, Athens gained and then lost control of large areas of central Greece. The conflict was concluded by the Thirty Years' Peace, which lasted until the end of the Pentecontaetia and the beginning of the Peloponnesian War.
The eventual breakdown of the peace was triggered by increasing conflict between Athens and several of Sparta's allies. Athens' alliance with Corcyra and attack on Potidaea enraged Corinth, and the Megarian decree imposed strict economic sanctions on Megara, another Spartan ally. These disputes, along with a general perception that Athenian power had grown too powerful, led to the breakdown of the Thirty Years Peace; the Peloponnesian War broke out in 431 BC.