History of the Peloponnesian War

For the adversary, finding himself now under obligation to repay the generosity in kind, instead of striving for vengeance for having had terms forced upon him, is moved by a sense of honour and is more ready to abide by his agreements.

Furthermore, men are more inclined to act thus toward their more serious enemies than toward those with whom they have had but trifling differences. And, finally, it is natural for men cheerfully to accept defeat at the hands of those who first make willing concessions, but to fight to the bitter end, even contrary to their better judgment, against an overbearing foe.