The Persian Wars began when Darius I of Persia sent forces to attack Athens in retaliation for its support of rebelling Greek cities. This led to the famous battles of Marathon and Thermopylae, where outnumbered Greeks held off the Persians. After the Persians were defeated at Salamis and Plataea, Athens emerged as the dominant city-state in Greece and organized the Delian League. However, this dominance led to conflict with Sparta and the outbreak of the Peloponnesian War in 431 BC, which devastated Greece and ended with Sparta capturing Athens in 404 BC.