The ancient federation known for its military strength and democratic government is the Delian League. Formed in the 5th century BCE, it was led by Athens and consisted of various Greek city-states. The federation was established primarily for mutual defense against Persian aggression following the Persian Wars. The Delian League allowed member states to contribute ships or money for a shared navy, enhancing collective security. However, over time, Athens' dominance led to increased tension and resentment among the other members, culminating in the Peloponnesian War against Sparta and its allies. This conflict ultimately highlighted the complexities of democracy and alliance in ancient Greece.