Fall of the Athenian Empire

What is now known as the Peloponnesian War was actually the second war between the Athenian and Spartan coalitions. The conflict between Athens and Sparta had its roots in the Persian Wars earlier in the fifth century B.C. After the Persian expedition led by Xerxes against Greece had been repulsed in 479, the Athenians assumed the leadership of the war against Persia in the Greek coastlands of Asia Minor. The Delian League, formed in 478 as an alliance against Persia, assumed the form of an empire as the Athenians began using force to prevent any of their "allies" from withdrawing from the League.
The First Peloponnesian War began in 460 when Megara withdrew from the Spartan alliance and allied itself with Athens. The Athenians built long walls for the Megarans to their port at Nisaea, thereby earning the everlasting enmity of Megara's old rival Corinth. At the height of their success in this war, the Athenians, allied with Megara and Argos, controlled all of Central Greece except for Thebes. Fortune turned against them in 454 when the destruction of a large Athenian force aiding an Egyptian revolt against Persia led to unrest and rebellions throughout the Athenian Empire. In 451 Athens and Sparta signed a "Thirty-Years Peace" in which Athens agreed to abandon its alliance with Argos, while Sparta promised to give up its alliance with Thebes. Athens was finally able to turn its full attention to the Persians, and in 449 Persia recognized the independence of the Greek cities and agreed to peace.