This was not an anti-Persian action, but a pragmatic assault on a native population that had lapsed into piracy. As a result of this action, the Athenians "liberated the Aegean", and they sent colonists to the island to prevent the island returning to piracy. Cimon returned a decade later to complete the expulsion of Persian forces from Europe. This action seems to have occurred concurrently with the Siege of Thasos, and so is generally dated to 465 BC. Evidently, even at this point, some Persian forces were holding (or had re-taken) some part of the Chersonesos with the help of native Thracians. Cimon sailed to the Chersonesos with just 4 triremes, but managed to capture the 13 ships of the Persians, and then proceeded to drive them out of the peninsula. Cimon then turned the Chersonesos (of which his father, Miltiades the Younger, had been tyrant before the Greco-Persian Wars began) over to the Athenians for colonisation. Once the Persian forces in Europe had largely been neutralised, the Athenians seem to have gone about starting to extend the League in Asia Minor. The islands of Samos, Chios and Lesbos seem to have become members of the original Hellenic alliance after Mycale, and presumably were also therefore original members of the Delian League. However, it is unclear exactly when the other Ionian cities, or indeed the other Greek cities of Asia Minor, joined the league, though they certainly did at some point. Cimon's Eurymedon campaign itself seems to have begun in response to the assembly of a large Persian fleet and army at Aspendos, near the mouth of the Eurymedon River. It is usually argued that the Persians were the would-be aggressors, and that Cimon's campaign was launched in order to deal with this new threat. Cawkwell suggests that the Persian build-up was the first concerted attempt to counter the activity of the Greeks since the failure of the second invasion. It is possible that internal strife with the Persian empire had contributed to the length of time it took to launch this campaign. Cawkwell suggests that the Persian forces gathered at Aspendos were aiming to move along the southern coast of Asia Minor, capturing each city, until eventually the Persian navy could begin operating in Ionia again. Plutarch says that upon hearing that the Persian forces were gathering at Aspendos, Cimon sailed from Cnidus (in Caria) with 200 triremes.