In the spring of 335, Alexander left Amphipolis, and by a rapid march of ten days reached Mount Hæmus in the thick of a population which had never recognized the supremacy of Macedon. They tried to defend themselves in their mountain passes, but Alexander soon forced his way through, and on the top of the highest mountain, celebrated his victory by setting up a thank offering to Dionysus. He then gave his attention to various mountain tribes with whom his father had had trouble, who had never before been subjugated, but who now met a decisive defeat at his hands. An island on the Danube, where the tribesmen had placed for security their wives and children and property, proved, however, impregnable. The young king showed himself from the first a master of strategy, for although he could not capture the island, he executed rapid movements along the river, beating the Getæ who were defending the passages, and when the Triballi had come to terms, he marched up the Danube, and then, crossing the eastern passes of the Hæmus range, returned to Pæonia. Alexander’s absence in the north in this untiring campaign against barbarian tribes, whose homes and habits were hardly known to the civilized states of Greece, was taken advantage of by his enemies. While he was fighting on the Danube, the King of Illyria, Kleitos, whose people had given trouble to Philip and whose father had fallen in battle with the Macedonians, rose in revolt. Several tribes farther north on the Adriatic coast joined with the Illyrians in this anti-Macedonian movement. Without a moment’s hesitation, Alexander turned to deal with his new enemies, and in order to do effective work, penetrated far into the mountainous region of Illyria. The Macedonian army soon found itself in a hazardous position, surrounded on all sides by hostile tribes. By skilful strategy, Alexander withdrew his troops from the danger that threatened them, while they were besieging Pelion in the face of superior numbers, and when he found that the Illyrians were following him, he quickly turned on them, administered a decisive blow, and forced Kleitos to seek a refuge in the territory of the Taulantines, one of the tribes which had been co-operating with the Illyrians in their resistance to his army.