The reference is Thucydides Book 1, ch. 124.
Quoted below is H. D. Cameron's comment, from his Thucydides Book I: A Student's Grammatical Commentary, p. 117. Cameron gives his translation in quotes.
εἴπερ βεβαιότατον τὸ ταὐτὰ ξυμφέροντα καὶ πόλεσι καὶ ἰδιώταις εἶναι. Real condition with present indicative (the missing copula ἐστί) indicating an admitted fact. "Given the fact that it is most secure for there to be the same interests for both cities and individuals [i.e., for cities and individuals to have the same interests]."
Given Cameron's comment, I'll be grateful for a discussion of two questions that occurred to me, but that I cannot properly answer.
1. *where* might "the missing copula ἐστί" be placed?
2. Is the attributive/predicative distinction relevant to the placement of "the missing copula ἐστί"? (My understanding of the attributive/predicative distinction in relation to word order is pretty shaky.)
In case there is need for more context I quote 1.124.1 in full, with a link to the Perseus presentation:
‘ὥστε πανταχόθεν καλῶς ὑπάρχον ὑμῖν πολεμεῖν καὶ ἡμῶν κοινῇ τάδε παραινούντων, εἴπερ βεβαιότατον τὸ ταὐτὰ ξυμφέροντα καὶ πόλεσι καὶ ἰδιώταις εἶναι, μὴ μέλλετε Ποτειδεάταις τε ποιεῖσθαι τιμωρίαν οὖσι Δωριεῦσι καὶ ὑπὸ Ἰώνων πολιορκουμένοις, οὗ πρότερον ἦν τοὐναντίον, καὶ τῶν ἄλλων μετελθεῖν τὴν ἐλευθερίαν, ὡς οὐκέτι ἐνδέχεται περιμένοντας τοὺς μὲν ἤδη βλάπτεσθαι, τοὺς δ᾽, εἰ γνωσθησόμεθα ξυνελθόντες μέν, ἀμύνεσθαι δὲ οὐ τολμῶντες, μὴ πολὺ ὕστερον τὸ αὐτὸ πάσχειν
http://www.perseus.tufts.edu/hopper/tex ... pter%3D124