E.C. Marchant, Commentary on Thucydides: Book 3

τό τε ναυτικόν—“a third reason is here given for the policy of the Athenians, of leaving the M. still independent,— namely, the apprehension of danger from driving them to unite their navy with that of the Peloponnesians” (Bloomfield).

καθ᾽ ἓν γενόμενον—standing alone this phrase would certainly mean ‘concentrating,’ ‘becoming united,’ as is proved by such passages as Xen. Hell. V. 2, 16 πῶς είκὸς ὑμᾶς τῆς μὲν Βοιωτίας ἐπιμεληθῆναι ὅπως μὴ καθ᾽ ἓν εἴη, πολὺ δὲ μείζονος ἁθροιζομένης δυνάμεως ἁμελῆσαι; This would give no point here, since the whole fleet is of course meant by τὸ ναυτικόν. The accepted meaning ‘uniting with others’ is only to be got through the addition, ἢ . . προσθέμενον, which must be regarded as an explanation. (Steup suggests that προσθέμενον should be removed, so that καθ᾽ ἓν γ. can be taken with the datives. I do not think it a likely note—in fact there is a scholium, ἐς ταὐτὸ ξυνελθόν, here—but I do think there is a difficulty, and possibly the first ή should be removed.)