A few years later Cersebleptes allied with Teres II and invaded the Chersonese, which was now under Macedon's protection. After asking the Persian king Artaxerxes III to cut the support of the Ionian towns for Cersebleptes, Philip finally felt confident enough to begin his most ambitious project so far: the conquest of inland Thrace in the form of a large campaign that would last from 342 to 340. Few details are known about this campaign. It seems to have started in May or June, when Philip's army pentetrated the interior by marching upstream the Martisa river. The Odrysians resisted valiantly and confronted the Macedonians in many battles. Philip faced several setbacks and even seems to have lost at least one battle. By the spring of 341, fighting was still raging and Philip was forced to call in reinforcements. Although detailed evidence is lacking he finally managed to improve his situation and defeated Cersebleptes and Teres at some point between the second half of 341 and the first half of 340.