A country of southeast Asia on the Gulf of Thailand. Once part of the Khmer empire that ruled the entire Mekong River valley, the area was controlled after the 15th century by the neighboring kingdoms of Siam and Annam. Cambodia became part of French Indochina in the 19th century and proclaimed its independence as a kingdom in 1953. After the overthrow of the monarchy in 1970, Cambodia commenced a long period of political and social disruption, especially under the Khmer Rouge regime led by Pol Pot, which was responsible for more than a million deaths from 1975 to 1979. The monarchy was restored in 1993. Phnom Penh is the capital and the largest city.