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

τὴν φυλακὴν . . εἶναι—Athens had at least one φρούριον at Salamis, and a few ships there maintained a not very efficient blockade of the port of Megara.

τούς τε Πελοποννησίους—parallel to τοῖς τε Μεγαρεῦσιν . . ἐσπλεῖν, and probably, like that clause, depending on τοῖς Ἀθηναίοις φυλακὴν εἶναι=τοὺς Ἀθηναίους φυλάσσεσθαι. This κατὰ σύνεσιν constr. cannot be paralleled in prose, but it would be ordinary in tragedy: see Jebb on Soph. Antig. 216. Thus τοὺς Π. is the anticipatory accus., for ὅπως οἰ Πελοποννήσιοι κτλ.: the differing constr., first ὅπως, then infin., after a single verb, is not unusual. (Several emendations have been proposed, of which πρός τε Πελ., depending on φυλακὴν εἶναι and τούς τε Πελ. σκοπῶν are worth notice.)

μὴ ποιῶνται . . αὐτόθεν—viz. from Nisaea, the harbour of Megara. The allusion in αὐτόθεν is not quite the same as in the previous case.

τὸ πρὶν γενόμενον—‘the previous incident’ occurred in the autumn of 429 B.C. The Pel. intended to surprise the Piraeus, but contented themselves with a descent on Salamis, which caused great alarm at Athens, and led at once to measures for the protection of the Athenian harbours (II. 93).