History of the Peloponnesian War

Both sides having been heard and the Athenian people twice assembled, in the former assembly they approved no less of the reasons of the Corinthians than of the Corcyraeans. But in the latter they changed their minds, not so as to make a league with the Corcyraeans both offensive and defensive, that the friends and enemies of the one should be so of the other (for then, if the Corcyraeans should have required them to go against Corinth, the peace had been broken with the Peloponnesians), but made it only defensive, that if anyone should invade Corcyra or Athens, or any of their confederates, they were then mutually to assist one another.

For they expected that even thus they should grow to war with the Peloponnesians and were therefore unwilling to let Corcyra, that had so great a navy, to fall into the hands of the Corinthians, but rather, as much as in them lay, desired to break them one against another; that if need required, they might have to do with the Corinthians, and others that had shipping, when they should be weakened to their hands.

And the island seemed also to lie conveniently for passing into Italy and Sicily.