Cambodia became a constitutional monarchy under King Norodom Sihanouk.
The area had been controlled by the Vietnamese since 1698 with King Chey Chettha II granting Vietnamese permission to settle in the area decades before.
In 1955, Sihanouk abdicated in favour of his father in order to be elected Prime Minister.
Soon the Khmer Rouge rebels would use him for gaining territory in the regions.
The King urged his followers to help in overthrowing the pro-United States government of Lon Nol, hastening the onset of civil war.
Some two million Cambodians were made refugees by the war and fled to Phnom Penh.
Estimates of the number of Cambodians killed during the bombing campaigns vary widely, as do views of the effects of the bombing.
The most outstanding prehistoric evidence in Cambodia however are probably "circular earthworks", discovered in the red soils near Memot and in adjacent region of Vietnam as of the end of the 1950s.
Their function and age are still debated, but some of them possibly date from 2nd millennium BC at least. The most part of evidence come from Khorat Plateau, Thai country nowadays.
The US Seventh Air Force argued that the bombing prevented the fall of Phnom Penh in 1973 by killing 16,000 of 25,500 Khmer Rouge fighters besieging the city.
Cambodia specialist Craig Etcheson argued that the Khmer Rouge "would have won anyway", even without US intervention driving recruitment although the US secretly played a major role behind the leading cause of the Khmer Rouge.
Upon his father's death in 1960, Sihanouk again became head of state, taking the title of Prince.