The Khmer people were among the first in Southeast Asia to adopt religious ideas and political institutions from India and to establish centralized kingdoms encompassing 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 (known as Siam until 1939). The golden age of Khmer civilization, however, was the period from the ninth to the thirteenth century, when the kingdom of Kambuja, which gave Kampuchea, or Cambodia, its name, ruled large territories from its capital in the region of Angkor in western Cambodia.

Under Jayavarman VII (1181-ca. 1218), Kambuja reached its zenith of political power and cultural creativity. Following Jayavarman VII's death, Kambuja experienced gradual decline. Important factors were the aggressiveness of neighboring peoples (especially the Thai, or Siamese), chronic interdynastic strife, 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 country.

The fifteenth to the nineteenth century was a period of continued decline and territorial loss. Cambodia enjoyed a brief period of prosperity during the sixteenth century because its kings, who built their capitals in the region southeast of the Tonle Sap (Great Lake) along the Mekong River, promoted trade with other parts of Asia. This was the period when Spanish and Portuguese adventurers and missionaries first visited the country. But the Thai conquest of the new capital at Lovek in 1594 marked a downturn in the country's fortunes and Cambodia became a pawn in power struggles between its two increasingly powerful neighbors, Siam and Vietnam. Vietnam's settlement of the Mekong Delta led to its annexation of that area at the end of the seventeenth century. Cambodia thereby lost some of its richest territory and was cut off from the sea. Such foreign encroachments continued through the first half of the nineteenth century because Vietnam was determined to absorb Khmer land and to force the inhabitants to accept Vietnamese culture. Such imperialistic policies created in the Khmer an abiding suspicion of their eastern neighbors that flared into violent confrontation after the Khmer Rouge established its regime in 1975.

In 1863 King Norodom signed an agreement with the French to establish a protectorate over his kingdom. The country gradually came under French colonial domination. During World War II, the Japanese allowed the French government (based at Vichy) that collaborated with the Nazis the Vichy French to continue administering Cambodia and the other Indochinese territories, but they also fostered Khmer nationalism. Cambodia enjoyed a brief period of independence in 1945 before Allied troops restored French control. King Norodom Sihanouk, who had been chosen by France to succeed King Monivong in 1941, rapidly assumed a central political role as he sought to neutralize leftist and republican opponents and attempted to negotiate acceptable terms for independence from the French. Sihanouk's "royal crusade for independence" resulted in grudging French acquiescence to his demands for a transfer of sovereignty. A partial agreement was struck in October 1953. Sihanouk then declared that independence had been achieved and returned in triumph to Phnom Penh. The following year, as a result of the Geneva Conference on Indochina, Cambodia was able to bring about the withdrawal of the Viet Minh troops from its territory and to withstand any residual impingement upon its sovereignty by external powers.

In order to play a more active role in national politics, Sihanouk abdicated in 1955 and placed his father, Norodom Suramarit, on the throne. Now only a prince, Sihanouk organized his own political movement, the Popular Socialist Community, (Sangkum Reastr Niyum, or Sangkum), which won all the seats in the National Assembly in the 1955 election. The Sangkum dominated the political scene until the late 1960s. Sihanouk's highly personal ruling style made him immensely popular with the people, especially in rural villages. Although the Sangkum was backed by conservative interests, Sihanouk included leftists in his government, three of whom--Khieu Samphan, Hou Yuon, and Hu Nim--later became leaders of the Khmer Rouge. In 1963 he announced the nationalization of banking, foreign trade, and insurance in a socialist experiment that dried up foreign investment and alienated the right wing. In foreign relations, Sihanouk pursued a policy of neutrality and nonalignment. He accepted United States economic and military aid, but he also promoted close relations with China and attempted to keep on good terms with the Democratic Republic of Vietnam (North Vietnam). The principal objectives of his foreign policy were to preserve Cambodia's independence and to keep the country out of the widening conflict in neighboring Vietnam. Relations with Washington grew stormy in the early 1960s. In 1963 the prince rejected further United States aid, and, two years later, he severed diplomatic relations.

Both the domestic and the international situations had deteriorated by the late 1960s. The increasingly powerful right wing challenged Sihanouk's control of the political system. Peasant resentment over harsh tax collection measures and the expropriation of land to build a sugar refinery led to a violent revolt in 1967 in the northwestern province of Batdambang (Battambang). The armed forces, commanded by General Lon Nol (who was also prime minister), quelled the revolt, but a communist-led insurgency spread throughout the country. The spillover of the Second Indochina War (or Vietnam War) into the Cambodian border areas also was becoming a serious problem. Apparently one factor in Sihanouk's decision to reestablish relations with Washington in 1969 was his fear of further incursions by the North Vietnamese and the Viet Cong. In March 1970, however, he was overthrown by General Lon Nol and other right-wing leaders, who seven months later abolished the monarchy and established the Khmer Republic.

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 struggle for independence and the creation in 1951, under Vietnamese auspices, of the Kampuchean (or Khmer) People's Revolutionary Party. During the early 1960s, however, a group of Paris-trained communist intellectuals, of whom the most important were Saloth Sar (known as Pol Pot after 1976), Khieu Samphan, and Ieng Sary, seized control of the party. They gradually purged or neutralized rivals whom they considered too subservient to Vietnam. After the March 1970 coup d'état that toppled Sihanouk, the Khmer Rouge formed a united front with the ousted leader, a move that won them the goodwill of peasants who were still loyal to the prince.

Despite massive United States aid to the newly proclaimed Khmer Republic and the bombing of North Vietnamese and Khmer Rouge installations and troop concentrations in the countryside, the Phnom Penh regime rapidly lost most of the country'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 country. Sixteen days later Khmer Rouge troops entered the city.

The forty-four months the Khmer Rouge were in power was a period 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 inclinations of local leaders, hundreds of thousands of people starved, died from disease, or were executed. "New people" (the intelligentsia and those from the cities--those new to the rural areas), being considered politically unreliable, were special targets of terror and of a harsh, unremitting regime of forced labor. In 1977 Pol Pot launched a bloody purge within the communist ranks that accounted for many deaths. The slaughter of the Vietnamese minority living in Cambodia and the Khmer Rouge's aggressive incursions into Vietnam led to fighting with Vietnam in 1977 and 1978. In December 1978, Vietnamese forces invaded the country. On January 7, 1979, they captured Phnom Penh and began to establish the People's Republic of Kampuchea. The Khmer Rouge fled to isolated corners of the country and resumed their guerrilla struggle, which continued in the late 1980s.