Approximate translation: Unless powers have nearly matched military strength, they cannot have equal or nearly equal voices in the deliberations of the alliance.
Some questions:
ὁμοῖόν τι ἢ ἴσον What is τι doing here?
τὸ κοινὸν βουλεύεσθαι Should this be understood as an articular infinitive with an adjective modifier?
Yes, and ὁμοῖόν τι ἢ ἴσον internal accusative with βουλεύεσθαι, “to take any (τι) equal or equitable counsel” (ες το κοινον “for the common good” v.sim., as katalogon).
Thanks to katalogon and mwh. Let me try again. This time I match up the Greek phrases with English phrases. I’m doing this because I had a lot of trouble with this sentence.