History of the Peloponnesian War

It was in this way that we acquired our empire—both we and all others that have ever won empire—by coming zealously to the aid of those, whether barbarians or Hellenes, who have at any time appealed to us; whereas, if we should all keep quiet or draw distinctions of race as to whom we ought to assist, we should add but little to our empire and should rather run a risk of losing that empire itself.

For against a superior one does not merely defend oneself when he attacks, but even takes precaution that he shall not attack at all. And it is not possible for us to exercise a careful stewardship of the limits we would set to our empire; but, since we are placed in this position, it is necessary to plot against some and not let go our hold upon others, because there is a danger of coming ourselves under the empire of others, should we not ourselves hold empire over other peoples.