In 335 BC, Alexander led a campaign against the Thracian tribe of the Triballi at Haemus Mons, fighting them along the Danube and forcing their surrender on Peuce Island. Shortly thereafter, the Illyrian king Cleitus of the Dardani threatened to attack Macedonia, yet Alexander took the initiative and besieged them at Pelion (in modern Albania). When Alexander was given news that Thebes had once again revolted from the League of Corinth and were besieging the Macedonian garrison in the Cadmea, Alexander left the Illyrian front and marched to Thebes, which he placed under siege. After breaching the walls, Alexander's forces killed 6,000 Thebans, took 30,000 inhabitants as prisoners of war, and burned the city to the ground as a warning to others, which proved effective since no other Greek state aside from Sparta dared to challenge Alexander for the remainder of his reign.