The Peloponnesian War began in 431 BCE as tensions grew between the Athenian Empire and the Peloponnesian League led by Sparta. A series of military campaigns and battles ensued over decades as each side tried to gain the upper hand. A key turning point was the disastrous Sicilian Expedition launched by Athens in 415 BCE, in which their entire expeditionary force was defeated and destroyed. This seriously weakened Athens and allowed Sparta to invade Attica. Athens ultimately surrendered in 404 BCE, losing its naval empire and political influence in Greece.