Cambodia's long history dates to 200 BC with the formation of kingdoms known collectively as Funan. Khmer kingdoms first appeared in the fifth century AD , the most famous of which is Angkor (ninth to fifteenth centuries). A Hindu-Buddhist kingdom, located in northwestern Cambodia, is the site of the famous temple complex known as Angkor Wat. At its peak in the twelfth century, the Khmer empire dominated much of present-day mainland Southeast Asia. Pressure from Thailand and Vietnam threatened the sovereignty of Cambodia in the nineteenth century, and King Ang Duong requested French protectorate status.
After 90 years of French colonial rule, Cambodia gained independence in 1953 under the leadership of Norodom Sihanouk. Since independence, Cambodia's history has been marked by civil war, foreign invasions, bloody revolution, and four types of political systems: a constitutional monarchy headed by Sihanouk (1953–70); a military-dominated republic led by General Lon Nol (1970–75); a communist regime led by Pol Pot (1975–79); and a socialist republic led by Hun Sen (1979–92).
The 1970–79 period was particularly brutal. Sihanouk was overthrown as head of state in a coup led by Lon Nol, paving the way for civil war. Simultaneously, Cambodia was dragged into the Vietnam War upon the invasion of U.S. and South Vietnamese troops. It is estimated that U.S. planes dropped 500,000 tons of bombs and destroyed countless villages and farms. When the Khmer Rouge emerged victorious in April 1975, the leadership of Pol Pot imposed a radical and murderous social revolution involving the systematic relocation, torture, and execution of educated Cambodians and urban residents. The capital of Phnom Penh became a ghost town as an estimated 2 million people (almost 25% of the total population) perished. Meanwhile, repeated border conflicts reached a climax in 1978 when Vietnam invaded Cambodia. In January 1979, Cambodia fell to the Vietnamese and Pol Pot's followers fled to the countryside to carry on their revolution.
From 1979 to 1992, the country was in a state of civil war with factions loyal to Pol Pot, the monarchy, and the Vietnam-installed regime. The Hun Sen period, however, is generally viewed as a significant improvement over the Lon Nol and Pol Pot regimes.