Lon Nol officially invited the United States to extend the Vietnam War into Cambodia. In its campaign to wreck the Ho Chi Minh supply trail from North to South Vietnam, which ran through Cambodia, the U.S. Air Force dropped 539,129 tons of bombs on Cambodia in 1969-73 (more than all of the bombs dropped on Japan in World War II), killing about 700,000 people, according to the CIA, and driving half of the rural population into the cities as refugees.
The bombing and flood of refugees led to the collapse of the agricultural system and induced a famine in which hundreds of thousands died from starvation. In fact, many of the deaths after 1975 that are attributed to the Khmer Rouge were actually caused by starvation from the famine induced by U.S. bombing before 1975. As the Khmer Rouge came to power in 1975, a U.S. Agency for International Development report estimated that it would likely take two to three years until Cambodia would regain its rice self-sufficiency.
Also not widely understood is that, according to CIA documents declassified in 1987, the U.S. bombing served to radicalize the population against the Lon Nol regime and helped the Khmer Rouge to move from being a politically weak and isolated movement in 1970 to having enough support to overthrow Lon Nol by 1975.
Despite the horrific record of the Khmer Rouge regime, this did not later prevent the Reagan Administration's policy "tilt" in favor of supporting the Khmer Rouge, once they were again rebels in the countryside, this time fighting to oust the Vietnam-backed Hun Sen government in Phnom Penh.
According to the "tilt" policy in Cold War logic, because the Khmer Rouge were fighting against the Vietnamese, and by association, the Soviets, it was acceptable for the United States to support them throughout the 1980s.