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Prelude to the war

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.

After consultations with its allies, Sparta sent a deputation to Athens demanding certain concessions, such as the immediate expulsion of the Alcmaeonidae family including Pericles and the retraction of the Megarian Decree, threatening war if the demands were not met. The obvious purpose of these proposals was the instigation of a confrontation between Pericles and the people; this event, indeed, would come about a few years later. At that time, the Athenians unhesitatingly followed Pericles' instructions. In the first legendary oration Thucydides puts in his mouth, Pericles advised the Athenians not to yield to their opponents' demands, since they were militarily stronger. Pericles was not prepared to make unilateral concessions, believing that "if Athens conceded on that issue, then Sparta was sure to come up with further demands". Consequently, Pericles asked the Spartans to offer a quid pro quo. In exchange for retracting the Megarian Decree, the Athenians demanded from Sparta to abandon their practice of periodic expulsion of foreigners from their territory (xenelasia) and to recognize the autonomy of its allied cities, a request implying that Sparta's hegemony was also ruthless. The terms were rejected by the Spartans, and with neither side willing to back down, the two cities prepared for war. According to Athanasios G. Platias and Constantinos Koliopoulos, professors of strategic studies and international politics, "rather than to submit to coercive demands, Pericles chose war". Another consideration that may well have influenced Pericles' stance was the concern that revolts in the empire might spread if Athens showed itself weak.