The History of the Grecian War (1.36.1-1.36.3)

If any man now think thus that what we have spoken is indeed profitable, but fears, if it were admitted, the league were thereby broken, let that man consider that his fear joined with strength will make his enemies fear, and his confidence, having (if he reject us) so much the less strength, will so much the less be feared. Let him also remember that he is now in consultation no less concerning Athens than Corcyra, wherein he forecasteth none of the best (considering the present state of affairs) that makes a question whether against a war at hand and only not already on foot he should join unto it or not that city which with most important advantages or disadvantages will be friend or enemy.

For it lieth so conveniently for sailing into Italy and Sicily that it can both prohibit any fleet to come to Peloponnesus from thence and convoy any coming from Peloponnesus thither, and is also for divers other uses most commodious.

And to comprehend all in brief, consider whether we be to be abandoned or not by this. For Greece having but three navies of any account, yours, ours, and that of Corinth, if you suffer the other two to join in one by letting the Corinthians first seize us, you shall have to fight by sea at one time both against the Corcyraeans and the Peloponnesians; whereas by making league with us, you shall, with your fleet augmented, have to deal against the Peloponnesians alone.