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

ξυμμαχία ἐγένετο—referring to the transference of the hegemony of the allies from Sparta to Athens in 476 B.C. (I. 95). In reality this transference was an ἀπόστασις from the Spartan alliance (Ath. Pol. c. 23); and the anxiety of Sparta to be quit of the war against Persia (ἀπαλλαξείοντες τοῦ Μηδικοῦ πολέμου I. 95) was not shown until after the transference took place. Samos, Chios and Lesbos were at the head of the movement in favour of Athens (Plut. Arist. c. 23). (This passage well illustrates the freedom with which Greek orators habitually manipulate history when they refer to the past.)

ἀπολιπόντων ἐκ—this use of άπολείπειν seems to result from such phrases as ἀπολείπειν τὴν ξυμμαχίαν (c 9), so that the full sense is ἀπολιπόντων (ἡμᾶς καὶ ἀπελθόντων).