Missing from your article is the U.S. role in the destabilization of Cambodia for refusing to become involved in the Vietnam War for the U.S. When Prince Norodom Sihanouk left the country, the U.S. installed their puppet Lon Nol. The bombings and incursions by the U.S. into Cambodia fueled the Khmer Rouge so by the time they came into Phnom Penh they were ready to exterminate anyone who they thought supported Lon Nol and the U.S.