I would go for the End of the Cold War
Explanation:
After the war and the victory of the communist forces, the South and North were united under the communist "dictatorship" of Hanoi...and yes the collapse of the US backed South led Pol Pot and the Khmer Rouge to power in Cambodia with consequent application of a kind of utopic policy of starting from zero and refusal of the past with consequent persecution of the "old" leading to atrocities and mass killings.
The War in Vietnam, and the defeat of the US backed forces of the South, had multiple effects and interfered with the stability of other nations nearby (Laos, Cambodia).
In an attempt to neutralize Cambodia as a base and supply route for the North and Vietcong communist forces the US started a military campaign against “sanctuaries” or enemy strongpoints in Cambodian territory.
On top of bombings areas of Cambodia and localized raids from Special Forces they “convinced” the army of South Vietnam to invade Cambodia in a disastrous campaign that precipitated the fall of the Cambodian government in favor of the Khmer Rouge.
Once in power and reasonably safe from external intervention (the South Vietnam was crumbling as well and the US only interest was to leave Vietnam) the Khmer leaders and Pol Pot in particular started a psychotic policy of abandoning the old in favor of a new social structure based upon abandoning towns and cities, discarding everything “occidental” (no medicines or hospital facilities were allowed) and weird huge collective enterprises such as dams, canals or cultivations without any planning, tools or even supplies for the workers. Exactly as in the Nazi concentration camps the “workers” were worked to death and starved. Any protest was met with execution under the suspicion of being an enemy of the state or a pro-western agitator.
