The entrance (of Piraeus) was unclosed and unguarded; as was natural, since the Athenians were complete masters of the sea. (Book 2 Chapter 93.1) - Thucydides

(Pericles:) If they attack our country by land, we shall attack theirs by sea;and the devastation, even of part of Peloponnesus, will be a very different thing from that of all Attica. For they, if they want fresh territory, must take it by arms, whereas we have abundance of land both in the islands and on the continent; such is the power which the empire of the sea gives. Reflect, if we were islanders, who would be more invulnerable? Let us imagine that we are, and acting in that spirit let us give up land and houses, but keep a watch over the city and the sea. We should not under any irritation at the loss of our property give battle to the Peloponnesians, who far outnumber us. […] Mourn not for houses and lands, but for men; men may gain these, but these will not gain men. If I thought that you would listen to me, I would say to you, “Go yourselves and destroy them, and thereby prove to the Peloponnesians that none of these things will move you.”

(Book 1 Chapter 143.4-5)

I blame not those who wish to rule, but those who are willing to serve. The same human nature which is always ready to domineer over the subservient, bids us defend ourselves against the aggressor.

You know well as I do that when we are talking on the human plane, questions of justice only arise when there is equal power to compel: in terms of practicality the dominant exact what they can and the weak concede what they must. (Said by Athenian envoy to the Melians)