At any point where they saw the Corcyraeans distressed, the Athenians appeared and kept the enemy in check; but the generals, who were afraid of diso… - Thucydides

Our opinion of the gods and our knowledge of men lead us to conclude that it is a general and necessary law of nature to rule whatever one can. This is not a law that we made ourselves, nor were we the first to act upon it when it was made. We found it already in existence, and we shall leave it to exist for ever among those who come after us. We are merely acting in accordance with it, and we know that you or anybody else with the same power as ours would be acting in precisely the same way.
Goodwill shown by the party that is asking for help does not mean security for the prospective ally. What is looked for is a positive preponderance of power in action.

In times of peace and prosperity cities and individuals alike follow higher standards, because they are not forced into a situation where they have to do what they do not want to do. But war is a stern teacher; in depriving them of the power of easily satisfying their daily wants, it brings most people’s minds down to the level of their actual circumstances.

It is useless to attack men who could not be controlled even if conquered, while failure would leave us in an even worse position.