The Khmer people were among the first in Southeast Asia to adopt religious ideas and political institutions from India and to establish centralized kingdoms comprehensive large territories. The earliest known kingdom in the area, Funan, flourished from around the first to the sixth century A.D. It was succeeded by Chenla, which controlled large areas of modern Cambodia, Vietnam, Laos, and Thailand. The golden age of Khmer civilization, was the time from the 9th to 13th century, when the kingdom of Kambuja, which gave Kampuchea, or Cambodia, its name, governed large territories from its capital in the region of Angkor in western Cambodia.

Under Jayavarman VII, Kambuja reached its zenith of political power and cultural creativity. Following Jayavarman VII's death, Kambuja practiced gradual decline. valuable factors were the aggressiveness of neighboring peoples, chronic interdynastic discord, and the gradual deterioration of the complex irrigation system that had ensured rice surpluses. The Angkorian monarchy survived until 1431, when the Thai captured Angkor Thom and the Cambodian king fled to the southern part of his nation.

The 15th to the 19th century was a time of continued decline and territorial loss. Cambodia enjoyed a brief time of prosperity during the 16th century because its kings, who built their capitals in the region southeast of the Tonle Sap along the Mekong River, promoted trade with other parts of Asia. This was the time when Spanish and Portuguese adventurers and missionaries first visited the nation.

The Khmer Republic faced not only North Vietnamese and Viet Cong combat units but also an effective, homegrown communist movement that grew more lethal as time went on. The Cambodian communists, whom Sihanouk had labeled Khmer Rouge, traced their movement back to the fight for freedom and the creation in 1951, under Vietnamese auspices, of the Kampuchean (or Khmer) People's Revolutionary Party. During the early 1960s, a group of Paris-trained communist intellectuals, of whom the most valuable were Saloth Sar, Khieu Samphan, and Ieng Sary, seized control of the party.

contempt large United States aid to the newly proclaimed Khmer Republic and the bombing of North Vietnamese and Khmer Rouge installations and troop concentrations in the nationside, the Phnom Penh regime rapidly lost most of the nation's territory to the communists. In January 1975 communist forces laid siege to Phnom Penh, and in succeeding months they tightened the noose around the capital. On April 1, 1975, President Lon Nol left the nation. Sixteen days later Khmer Rouge troops entered the city.

The forty-four months the Khmer Rouge were in power was a time of unmitigated suffering for the Khmer people. Although the severity of revolutionary policies varied from region to region because of ideological differences and the personal tendency of local leaders, hundreds of thousands of people starved, died from disease, or were executed.