Plato, Laws 684b

ἑτέροις, βοηθήσειν δὲ βασιλῆς τε βασιλεῦσιν ἀδικουμένοις καὶ δήμοις, καὶ δῆμοι δήμοις καὶ βασιλεῦσιν ἀδικουμένοις. ἆρʼ οὐχ οὕτως; οὕτω μὲν οὖν. οὐκοῦν τό γε μέγιστον ταῖς καταστάσεσιν τῶν πολιτειῶν ὑπῆρχεν ταῖς ἐν ταῖς τρισὶ πόλεσι νομοθετουμέναις, εἴτε οἱ βασιλῆς ἐνομοθέτουν εἴτʼ ἄλλοι τινές; ποῖον; τὸ βοηθούς γε εἶναι τὰς δύο ἐπὶ τὴν μίαν ἀεὶ πόλιν, τὴν τοῖς τεθεῖσιν νόμοις ἀπειθοῦσαν. δῆλον.

when wronged, and the peoples aid both peoples and kings. Was not that the way of it? It was. In the polities legally established—whether by the kings or others—in the three States, was not this the most important principle? What? That the other two States should always help against the third, whenever it disobeyed the laws laid down. Evidently. And surely most people insist on this,—