Pericles was convinced that the war against Sparta, which could not conceal its envy of Athens' pre-eminence, was inevitable if unfortunate. Therefore, he did not hesitate to send troops to Corcyra to reinforce the Corcyraean fleet, which was fighting against Corinth. In 433 BC the enemy fleets confronted each other at the Battle of Sybota and a year later the Athenians fought Corinthian colonists at the Battle of Potidaea; these two events contributed greatly to Corinth's lasting hatred of Athens. During the same period, Pericles proposed the Megarian decree, which resembled a modern trade embargo. According to the provisions of the decree, Megarian merchants were excluded from the market of Athens and the ports in its empire. This ban strangled the Megarian economy and strained the fragile peace between Athens and Sparta, which was allied with Megara. According to George Cawkwell, a praelector in ancient history, with this decree Pericles breached the Thirty Years' Peace "but, perhaps, not without the semblance of an excuse". The Athenians' justification was that the Megarians had cultivated the sacred land consecrated to Demeter and had given refuge to runaway slaves, a behavior which the Athenians considered to be impious.