Thucydides 3.82.2 (tr. C.F. Smith):
For in peace and prosperity both states and individuals have gentler feelings, because men are not then forced to face conditions of dire necessity; but war, which robs men of the easy supply of their daily wants, is a rough schoolmaster and creates in most people a temper that matches their condition.
ἐν μὲν γὰρ εἰρήνῃ καὶ ἀγαθοῖς πράγμασιν αἵ τε πόλεις καὶ οἱ ἰδιῶται ἀμείνους τὰς γνώμας ἔχουσι διὰ τὸ μὴ ἐς ἀκουσίους ἀνάγκας πίπτειν· ὁ δὲ πόλεμος ὑφελὼν τὴν εὐπορίαν τοῦ καθ᾿ ἡμέραν βίαιος διδάσκαλος καὶ πρὸς τὰ παρόντα τὰς ὀργὰς τῶν πολλῶν ὁμοιοῖ.