E.C. Marchant, Commentary on Thucydides Book 1

20 ἐπαρθέντας αὐτῷ—prompted by that expectation; Eur. Orest. 286 ὅστις μ᾽ ἐπάρας ἔργον ἀνοσιώτατον (to a ...) κτλ.

22. ὑπαρχούσης πρότερον—that existed already. This is the Engl. equivalent, though ὑπαρχούσης is really imperf. partic., past in reference to ὑφείλομεν. ὑφελεῖν μᾶλλον is to diminish rather than to add to, and the gen. is partitive (μᾶλλον does not belong to σῶφρον as Classen took it). For πρότερον ὐπῆρχε cf. vii. 28 πόλεμον οὐδὲν ἐλάσσω προσανείλοντο τοῦ πρότερον υ:πάρχοντος. The transl. ‘that has existed for some time’ is simple, but scarcely justified.

διὰ Μεγαρέας—what event is alluded to? Edd. are much divided between (1) ‘the Megarian decree,’ by which Athens excluded Megara from all her ports and markets (c. 67); (2) the revolt of Megara to Athens after a dispute with Corinth (c. 103 for τὸ σφοδρὸν μῖσος that Corinth conceived for Athens on this account) in 465 B.C.; (3) the revolt of Megara with Corinthian support, from Athens in 445 B.C. (cc. 114-115). The ὑποψία is that felt by Corinth, so that (3) appeais unlikely—note ἔγκλημα. As (1) is the only one of these events that had happened since the thirty years' truce. it is the most probable; but the date of the Megarian decree is unfortunately doubtful, and some suppose, on insufficient evidence, that it was not passed so early as 433 B.C.