It is important to remember that Cambodian history did not begin with the Khmer Rouge. Pol Pot's incredibly harsh regime has garnered most attention, but the Cambodians enjoy a long and often triumphant history. Anybody who witnesses the magnificence temples at Angkor will be able to see that the Khmer Empire was once a wealthy, militarized, and a major force in the region. Its zenith came under Jayavarman VII (1181-ca. 1218), where the Empire made significant territorial gains from the Vietnamese and Cham. The Khmer Empire stretched from modern day Thailand, Malaysia, Burma, Laos and Vietnam.
The period following the fall of the Khmer Empire has been described as Cambodia's dark ages.
French colonial expansion in the area known then as Indochina included coming to dominate Cambodia as a protectorate under French political control. However, the French were always more concerned with their possessions in Vietnam. Education of Cambodians was neglected for all but the established Elite. It was from this elite that many "Red Khmers" would emerge. Japan's hold on Southeast Asia during the Second world War undermined French prestige and following the Allied victory Prince Sihanouk soon declared independence. This was a relatively peaceful transition; France was too absorbed with its struggle in Vietnam, which it saw as more important to its conception of L'Indochine Francaise.
Prince Sihanouk was the main power figure in the country after this. He was noted for making very strange movies in which he starred, wrote and directed. His rule was characterized at this point with a Buddhist revival and an emphasis on education. This was a mixed blessing however. He succeeded in making an educated elite who became increasingly disenchanted with the lack of jobs available. As the economic situation in Cambodia deteriorated, many of these young people were attracted to the Indochinese Communist Party, and later the Khmer Rouge.
Second Indochina War
As the Second Indochina War spread to Cambodia's border (an important part of the "Ho Chi Minh trail"), the USA became increasingly concerned with events in the country. While traveling to Moscow and Beijing, Sihanouk was overthrown by Lon Nol and other generals who were looked upon favorably by the United States. Sihanouk then put his support behind the Khmer Rouge. This change influenced many to follow suit; he was after all considered a Boddhisatva. Meanwhile the Khmer Rouge followed the Vietnamese example and began to engender themselves to the rural poor.
Communist Khmer Rouge
Following a five-year struggle, Communist Khmer Rouge forces captured Phnom Penh in 1975 and ordered the evacuation of all cities and towns. Over 1 million people (and possibly many more) died from execution or enforced hardships. Those from the cities were known as "new" people and suffered worst at first. The rural peasantry were regarded as "base" people and fared better. However, the Khmer Rouge's cruelty was enacted on both groups. It also depended much upon where you were from. For example, people in the East generally got it worse. It is debated whether or not the Khmer Rouge began "crimes against humanity" or a protracted "genocide".
There are claims that what is clear, as Ben Kiernan argues, there was a disproportionate number of ethnic Chams killed, and the ethnically Vietnamese also suffered persecution. Nonetheless, the Khmer also suffered often indescriminate mass killings. A 1978 Vietnamese invasion drove the Khmer Rouge into the countryside and ended 13 years of fighting (but the fighting would continue for some time in in border areas).
Cold War Politics
Cold War politics meant that despite the horrendous crimes committed by the Khmer Rouge they were the recognized government long after the liberation of the country by the Vietnamese, indeed they continued to receive covert support and financing by the USA. As a result of the devastating politics of the Khmer Rouge regime, there was virtually no infrastructure left. Institutions of higher education, money, and all forms of commerce industries were destroyed in 1978, so the country had to be built up from scratch. UN-sponsored elections in 1993 helped restore some semblance of normalcy, as did the rapid diminution of the Khmer Rouge in the mid-1990s. A coalition government, formed after national elections in 1998, brought renewed political stability and the surrender of remaining Khmer Rouge forces.
The International Criminal Court is currently putting Leng Sary, Pol Pot's brother in law, on trial for 'crimes against humanity'.