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

πειθόμενοι μέν—the steps of the argument, which is difficult to follow to ἀνδραγαθίζεσθαι and has given rise to much discussion, are as follows: (1) if you punish M., you will act as Justice and Interest alike demand; (2) it may be said that Justice is on their side and against you (εἰ γάρ); (3) even if it is, you must follow the dictates of Interest (εἰ δὲ δή).

οὐ χαριεῖσθε—because they will still hate you.

δικαιώσεσθε—‘you will punish yourselves’ (instead of them) by proclaiming to all that you have no right to exert authority. εἰ γάρ—γάρ does not introduce the reason for the preceding statement, but = ‘it is true that,’ as in c. 43, 5 (Stahl in Rhein. Mus. 1901). οὐ χρεών is acc. abs. On the form of condition (cf. I. 38, VI. 92) Goodwin points out that it is “a perfectly natural combination, each part having its proper force.” If they were justified, you must have no right to your empire: εἰ δὲ δὴ . . κολάζεσθαι (mid ) proceeds on this assumption. If you persist in ruling (τοῦτο δρᾶν), even though you have no right to do so, then your interests require that you punish them, as you have others. The only alternative (ἤ=‘otherwise’) is, you must give up empire; you must run no risks (ἐκ τοῦ ἀκινδύνου=ἀκινδύνως) and play an honourable part.

παύεσθαι . . ἀνδραγαθίξεσθαι — Cleon echoes words attributed to Pericles at II. 63. ἀνδραγαθίζομαι was, it would seem, in use among the aristocrats who favoured a peace policy; cf. for the noun with ‘a political or social reference’ c. 57, 1, 64, 4 (Neil, Append. to Aristoph. Eq.).