Plato, Laws 626a

ἀρχομένους διακεκοσμημένους εἶναι φύλακας αὐτῶν, τοῦτο καὶ ἐν εἰρήνῃ δραστέον. ἣν γὰρ καλοῦσιν οἱ πλεῖστοι τῶν ἀνθρώπων εἰρήνην, τοῦτʼ εἶναι μόνον ὄνομα, τῷ δʼ ἔργῳ πάσαις πρὸς πάσας τὰς πόλεις ἀεὶ πόλεμον ἀκήρυκτον κατὰ φύσιν εἶναι. καὶ σχεδὸν ἀνευρήσεις, οὕτω σκοπῶν, τὸν Κρητῶν νομοθέτην ὡς εἰς τὸν πόλεμον ἅπαντα δημοσίᾳ καὶ ἰδίᾳ τὰ νόμιμα ἡμῖν ἀποβλέπων συνετάξατο, καὶ κατὰ ταῦτα

they must be carried out equally in time of peace. For (as he would say) peace, as the term is commonly employed, is nothing more than a name, the truth being that every State is, by a law of nature, engaged perpetually in an informal war with every other State. And if you look at the matter from this point of view you will find it practically true that our Cretan lawgiver ordained all our legal usages, both public and private, with an eye to war, and that he therefore charged us with the task of guarding our laws safely,