E.C. Marchant, Commentary on Thucydides: Book 2
ἄρχεται—not historic, but primary and connected with γέγραπται below. Thuc. means to say ‘what preceded (i.e. book I.) was an introduction: now begins (my account of) the war itself.’ Cf.ἀρξάμενος, I. 1, 1: Diod. XII. 37 τὴν ἱστορίαν ἐντεῦθεν ἀρξάμενος. Ullrich first rightly explained this passage.
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