C.E. Graves, Commentary on Thucydides: Book 4
τοῦ δ᾽ ἐπιγιγνομένου θέρους—the opening words introduced by δέ complete the sentence which ends the preceding book, ταῦτα μὲν κατὰ τὸν χειμῶνα τοῦτον ἐγένετο κ.τ.λ. The third and fifth books begin in the same way. θέρους—‘in the summer’; the genitive denotes the time within the limits of which a thing occurs, and is partitive in character (Madvig, § 66). Thucydides divides his history into summers and winters, thus reckoning more accurately, as he points out (v. 20), than by the names of the archons or other officials of the year in different states.
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