paragraph: Before crossing to Asia, Alexander wanted to safeguard his northern borders. In the spring of 335 BC, he advanced to suppress several revolts. Starting from Amphipolis, he traveled east into the country of the "Independent Thracians"; and at Mount Haemus, the Macedonian army attacked and defeated the Thracian forces manning the heights. The Macedonians marched into the country of the Triballi, and defeated their army near the Lyginus river (a tributary of the Danube). Alexander then marched for three days to the Danube, encountering the Getae tribe on the opposite shore. Crossing the river at night, he surprised them and forced their army to retreat after the first cavalry skirmish. News then reached Alexander that Cleitus, King of Illyria, and King Glaukias of the Taulanti were in open revolt against his authority. Marching west into Illyria, Alexander defeated each in turn, forcing the two rulers to flee with their troops. With these victories, he secured his northern frontier. While Alexander campaigned north, the Thebans and Athenians rebelled once again. Alexander immediately headed south. While the other cities again hesitated, Thebes decided to fight. The Theban resistance was ineffective, and Alexander razed the city and divided its territory between the other Boeotian cities. The end of Thebes cowed Athens, leaving all of Greece temporarily at peace. Alexander then set out on his Asian campaign, leaving Antipater as regent. 
question: What two groups did Alexander defeat in order to secure the northern frontier?
answer: Illyria