Short history of modern Cambodia
First administration of Sihanouk: 1955-1970
He started a political faction called the People's Socialist Community (Sangkum Reastr Niyum) which then won by a landslide in the 1955 national elections. In part the success was due to his popularity, but also from police brutality at the polling stations.
In 1960, when his father died he was named head of state (up until then he'd been the prime minister). Although he had remained neutral in a struggle between the US and USSR regarding tensions in Vietnam, he changed his position in 1965 and eliminated diplomatic relations with the US.
At the same time he allowed the Communist Vietnamese access to Cambodian soil to set up bases. With the Cambodian economy becoming unstable, Sihanouk decided to renew his relations with the US, who were secretly planning on bombing Cambodian areas suspected of housing Vietnamese Communists.
The Khmer Republic and the War: 1970-1975
To add to Lon Nol's problems, Sihanouk had been persuaded to set up a government while in exile, called the Khmer Rouge. The Khmer Rouge became a thorn in Lon Nol's side along with the Vietnamese until the Khmer regime collapsed. Another contributing factor to the collapse was the repeated US bombing of the Cambodian countryside. In 1975, the Khmer Rouge was able to take over Phnom Penh and shortly thereafter, the North Vietnamese were occupying South Vietnam.
The Khmer Rouge felt antipathy toward Cambodians living in urban areas and forced them to the countryside where they were forced to work in various forms of agriculture. Leading the Khmer Rouge was a man by the name of Saloth Sar, better known as Pol Pot. The government, Democratic Kampochea (DK), was run in part by rural Cambodians who were illiterate, but had fought along with the Khmer Rouge in the war.
Democratic Kampuchea (the Khmer Rouge/Red Khmer age): 1975-1979
The Khmer Rouge's plan to attack Vietnam and other areas backfired when the Vietnamese surprised Cambodia with an attack of over 100,000 troops. They were accompanied by Cambodian Communist rebels and managed to invade Phnom Penh, which had been vacated by the Khmer Rouge the day before.
The Khmer Rouge, Pol Pot among them, fled to the Thai-Cambodian border, where they were given asylum by the Thai government, which was unfriendly to Vietnam.
People's Republic of Kampuchea (Vietnamese occupation): 1979-1993
The UN upheld this government in exile, with the support given to it by the US, China and Thailand. With more ensuing conflicts between the two governments, many of Cambodia's finest along with the general population, totaling over half a million people, resettled in other countries.
By the end of 1989, the Cold War had ended which had the Vietnamese exiting Cambodia. Without financial support from the Soviets, the Vietnamese couldn't keep their troops in the country.
This withdrawal made things difficult for Cambodians, especially the prime minister, Hun Sen. The Khmer Rouge had not disappeared, but had made their presence known and were threatening military action. Since Cambodia was without much needed foreign aid, they discarded socialism and tried to get investors interested in the country.
Another major change was in the country's name, it was changed to the State of Cambodia (SOC), while the KPRP (who currently ruled Cambodia) changed their name to the Cambodian People's Party. An attempt to have a free-market economy just increased the gap between the rich and the poor with many government officials becoming millionaires.
In 1991, the UN, Cambodia, and other interested parties came to an agreement to end the Cambodian conflict. A United Nations Transitional Authority (UNTAC) and a Supreme National Council (SNC) were formed and were comprised of members from different factions within Cambodia. The agreement in Paris and the UN protectorate started competitive politics in Cambodia, something they hadn't seen for about 40 years.
Modern Cambodia: 1993-Present
A name change for the country was in order, so in 1993 Cambodia became known as the Kingdom of Cambodia and Sihanouk became the king once again after ratifying a new constitution which re-established the monarchy. After these changes were made, the UN no longer accepted the DK as the ruling party, thus causing them (the DK) to lose their seat and power in the UN.
The tentative compromise between the FUNCINPEC and the CPP fell apart in 1997 when Prince Ranariddh was overseas. Hun Sen took advantage of the Prince's absence and organized a violent takeover to replace him. He replaced Prince Ranariddh with another member of the FUNCINPEC, but this time with one who was more easily manipulated and compliant. In spite of this takeover, the elections of 1998 were carried out, but not without foreign observations.
Although it was stated the voting was fair, the CPP hassled it's opposition and following the elections many were put in jail while a few others were killed. Once again, the results were not accepted, but this time it was Prince Ranariddh who opposed it. Yet again another compromise was reached with Hun Sen as the only prime minister and with Prince Ranariddh as the president of the national assembly.
Things are stabilizing in Cambodia, but not without the help and support of foreign aid. With the outside world's interest waning, it's help is steadily decreasing discouraging any hopes for economic advancement and democracy.