Harold North Fowler, Commentary on Thucydides Book 5

ἀνάγκη γάρ: sc. τὸ ξυμφέρον λέγειν.—οὕτω...λέγειν: is the obj. of ὑπέθεσθε: “to talk so (as you have just been doing) not about justice, but about what is advantageous.”

ὑπέθεσθε: i.e. ὑπόθεσιν ἐποιήσασθε, “you have made it the foundation (or the startingpoint) of the discussion.” This use occurs in Thuc. only here, but freq. in Plat. (e.g. Charm. 171 d, ὃ ἐξ ἀρχῆς ὑπετιθέμεθα; Rep. iv. 437 a) and later writers. A similar use with added dat. is found in Hdt.i. 156. 2, ταῦτά οἱ ὑπετίθετο; iv. 135. 12; v. 98. 10; vii. 237. 10.—τὸ κοινὸν ἀγαθόν: what is meant is τὸ δίκαιον as the foundation of human society (see on c. 104. 4), which has, however, been excluded from the discussion.—ἀλλὰ τῷ κτἑ.: “yetitis expedient (χρήσιμον embraces τῷ ἀεὶ . . . δίκαια as well as the other infs.) that to every one in peril what is reasonable be accounted right”; i.e. even if we must not call it a right (δίκαιον), still it should have the effect of a right. St. strikes out δίκαια in 5, but by so doing he loses the point of the subtle and artificial argumentation of the Melians by which, as they dare not openly oppose the Athenians, they try to substitute εἰκότα for δίκαια. (καί, which some Mss. insert before δίκαια, is inappropriate.)

ἐντὸς τοῦ ἀκριβοῦς πείσαντα: the aor. for πείσοντα of most Mss. seems necessary; for it must refer to time antecedent to ὠφεληθῆναι, “even if one has not entirely proved his point, even if one has not attained to the ἀκριβές（ἐντός, like Lat. citra and sometimes intra, denotes that a missile has fallen short of the mark), still it is expedient (χρήσιμον) that one derive some advantage.” See App.

πρὸς ὑμῶν: in your interest. Cf. ii. 86. 19; iii. 38. 3. The reason for this is very cautiously expressed. Instead of saying: your contempt of justice will sometime cost you dear; for if those whom you now oppress ever come into power, they will take such revenge upon you that your fate will be a warning example to other ruling states, the Melians express the same idea in the intentionally obscure form, inasmuch as you, if you should ever fall, would, by the heaviest vengeance (which you will then incur), be a (warning) example to others.— ἐπὶ μεγίστῃ τιμωρίᾳ: the prep. prop. denotes the accompanying circumstances or conditions. Cf. ἐπὶ τοῖς δεινοῖς εὐέλπιδες, i. 70. 10; 143. 8; iii. 67. 33; iv. 22. 13.