It used to be nice here. Here's what happened.
1864: Cambodia is conquered by France.
1953: France is distracted by unrest in Vietnam, so King Sihanouk declares independence.
1959: Vietnam War begins: North Vietnamese communists (Viet Cong) backed by Soviet Union and China vs South Vietnamese fascists backed by United States.
Viet Cong use Cambodia as a hideout and transport route, drawing Cambodia into the war. Cambodian communists form the Khmer Rouge in support.
1969: US commences carpet-bombing Cambodia, hoping to hit hiding Viet Cong. Civilian casualties increase Khmer Rouge recruitment.
1970: General Lon Nol, with US support, deposes King Sihanouk. Sihanouk allies with the Khmer Rouge.
1970-1975: Civil war: Viet Cong + Khmer Rouge, supported by Sihanouk, vs Lon Nol's Government troops, supported by the US. The average Cambodian is forced to choose between King and Country.
1975: North Vietnamese win the Vietnam War.
1975: Khmer Rouge win the civil war. They immediately attempt to create an agrarian utopia by killing all dissenters and educated people. Through starvation, disease, and execution, around 40% of the population is killed.
The Khmer Rouge launches attacks into Vietnam, despite Vietnam being a former ally, hoping to reclaim territory.
1979: Vietnam retaliates against Cambodia and conquers it in 13 days.
1979-1989: Civil war: Khmer Rouge (in hiding) vs Vietnam (occupying) + Cambodian supporters (led by Hun Sen). The average Cambodian is forced to choose between the murderous, fascist Khmer Rouge and the foreign Vietnamese.
1989: Out of money, the Vietnamese leave. The UN takes over and organises a peace treaty and elections.
Hun Sen forms the Cambodian People's Party.
1993: Elections won by FUNCINPEC (led by Prince Ranirdh, and supported by the Khmer Rouge). The CPP refuses to accept the result. The UN brokers a deal: there will be two prime ministers.
The CPP continues to skirmish with the KR. Hun Sen (prime minister #1) slowly erodes Prince Ranirdh's (prime minister #2) power.
1997: The KR collapses through internal splits and defections, becoming part of FUNCINPEC.
1997: The CPP takes power in a coup. With nobody left to fight, Cambodia enters a period of relative peace.
There are two obvious places where things could have gone differently.
The first was getting involved in the American War (or the Vietnam War as it's known over there). This led to bombing and a coup which resulted in two civil wars.
The second was the brutal rule of the Khmer Rouge in the period between the wars. They started as a popular revolution; Phnom Penh cheered when they arrived. But then the leadership went absolutely, murderously insane - there's no other word for it; their actions weren't even in their own interests. I still don't understand how this happened so quickly.