The war began over a dispute between Corcyra and Epidamnus. Corinth intervened on the Epidamnian side. Fearful lest Corinth capture the Corcyran navy (second only to the Athenian in size), Athens intervened. It prevented Corinth from landing on Corcyra at the Battle of Sybota, laid siege to Potidaea, and forbade all commerce with Corinth's closely situated ally, Megara (the Megarian decree).