Laos traces its history to the kingdom of Lan Xang ('million elephants'), which was founded in the 14th century by a Lao prince, Fa Ngum, whose father had his family exiled from the Khmer Empire. Fa Ngum, with 10,000 Khmer troops, conquered many Lao principalities in the Mekong river basin, culminating in the capture of Vientiane. Ngum was descended from a long line of Lao kings that traced back to Khoun Boulom. He made Theravada Buddhism the state religion, and Lan Xang prospered. His ministers, unable to tolerate his ruthlessness, forced him into exile to the present-day Thai province of Nan in 1373, where he died. Fa Ngum's eldest son, Oun Heuan, ascended to the throne under the name Samsenethai and reigned for 43 years. Lan Xang became an important trade centre during Samsenthai's reign, but after his death in 1421 it collapsed into warring factions for nearly a century.