However, at the approach of spring (B.C. 335), he marched towards Thrace, into the lands of the Triballians and Illyrians,[21] because he ascertained that these nations were meditating a change of policy; and at the same time, as they were lying on his frontier, he thought it inexpedient, when he was about to start on a campaign so far away from his own land, to leave them behind him without being entirely subjugated. Setting out then from Amphipolis, he invaded the land of the people who were called independent Thracians,[22] keeping the city of Philippi and mount Orbelus on the left. Crossing the river Nessus,[23] they say he arrived at mount Haemus[24] on the tenth day. Here, along the defiles up the ascent to the mountain, he was met by many of the traders equipped with arms, as well as by the independent Thracians, who had made preparations to check the further advance of his expedition by seizing the summit of the Haemus, along which was the route for the passage of his army. They had collected their waggons, and placed them in front of them, not only using them as a rampart from which they might defend themselves, in case they should be forced back, but also intending to let them loose upon the phalanx of the Macedonians, where the mountain was most precipitous, if they tried to ascend. They had come to the conclusion[25] that the denser the phalanx was with which the waggons rushing down came into collision, the more easily would they scatter it by the violence of their fall upon it.