The Peloponnesian League was a military alliance of city-states in ancient Greece, primarily located in the Peloponnese region. Formed in the 6th century BCE, it was led by Sparta and included various allies who sought to protect their interests against external threats, particularly from the Delian League, which was led by Athens. The league played a significant role in the Peloponnesian War (431-404 BCE), a conflict between the Spartan and Athenian alliances. The Peloponnesian League's strength lay in its unified military strategy, which ultimately contributed to the defeat of Athens and the end of its dominance in Greece.