The Peloponnesian League, led by Sparta, defeated Athens in the Peloponnesian War.
The conflict formerly began in 431 BCE following the collapse of the previous Thirty Years’ peace between Athens and Sparta. As the two dominant powers in Greece, the city-states each led alliances of other independent territories in the area. The Athenian empire had the stronger navy due its location on the Aegean Sea, while the Peloponnesian League under Sparta had the stronger land army.
The contemporary historian Thucydides wrote an extensive account of the tensions between the two powers, and claimed that the war began due to Sparta’s concern at the growing power of Athens. In 431 BCE Sparta launched the first of many invasions of Attica under the command of the Spartan king Archidamos. This signalled the start of the first of three phases of the Peloponnesian War that engulfed much of the Greek world and left the Athenian empire in tatters.
During the first phase of the war, known as the Archidamian War, Sparta benefitted from an outbreak of the plague in Athens. This killed the leading Athenian general, Pericles, along with up to two-thirds of the population of the city. Although Athens went on to mount some successful offensives the two sides agreed to the uneasy Peace of Nicias in 421 BCE.
The war resumed in 415 BCE, after which Athens launched a disastrous attack on Syracuse in Sicily. Subsequent years saw Sparta build its own naval fleet with financial support from Persia, and it was with this that they were able to destroy Athens’ fleet at Aegospotami 405 BCE. Athens surrendered the following year, dramatically altering the balance of power in Greece.