In 335 BC, Alexander fought against the Thracian tribe of the Triballi at Haemus Mons and along the Danube, forcing their surrender on Peuce Island. Shortly thereafter, the Illyrian king Cleitus of the Dardani threatened to attack Macedonia, but Alexander took the initiative and besieged the Dardani at Pelion (in modern Albania). When Thebes had once again revolted from the League of Corinth and was 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 that convinced all other Greek states except Sparta not to challenge Alexander again.