About the whole thing being an infinitive phrase, that's because it's indirect discourse, continuing from the previous line. So it represents what the Thebans said and not what the author is saying -- notice also how the μέν here goes with the following δέ in the direct quote. But this sort of implicit indirect discourse is very common.
For ταὐτὰ ἐγίνωσκον, γιγνώσκω can also mean "think", so here's it's "thought the same things" = "had the same opinion".
So my close translation would be: for it is difficult, they said, even for all men to overcome by force the Greeks being of the same mind, who had formerly thought the same things.
(I couldn't find a good literal translation of the participle, but I'm guess you're okay with that part.)