http://www.pbs.org/frontlineworld/stories/cambodia/tl02.html1969-1974
Caught In The Crossfire
On March 18, 1969, American B-52s began carpet-bombing eastern Cambodia. "Operation Breakfast" was the first course in a four-year bombing campaign that drew Cambodia headlong into the Vietnam War. The Nixon Administration kept the bombings secret from Congress for several months, insisting they were directed against legitimate Vietnamese and Khmer Rouge targets. However, the raids exacted an enormous cost from the Cambodian people: the US dropped 540,000 tons of bombs , killing anywhere from 150,000 to 500,000 civilians.
Shortly after the bombing began, Sihanouk restored diplomatic relations with the US, expressing concern over the spread of communism in Southeast Asia. But his change of heart came too late. In March 1970, while Sihanouk was traveling abroad, he was deposed by a pro-American general, Lon Nol. The Nixon Administration, which viewed Sihanouk as an untrustworthy partner in the fight against communism , increased military support to the new regime.
In April 1970, without Lon Nol's knowledge, American and South Vietnamese forces crossed into Cambodia. There was already widespread domestic opposition to the war in Vietnam; news of the "secret invasion" of Cambodia sparked massive protests across the US, culminating in the deaths of six students shot by National Guardsmen at Kent State University and Jackson State University. Nixon withdrew American troops from Cambodia shortly afterwards. But the US bombing continued until August 1973.
Meanwhile, with assistance from North Vietnam and China, the guerrillas of the Khmer Rouge had grown into a formidable force. By 1974, they were beating the government on the battlefield and preparing for a final assault on Phnom Penh. And they had gained an unlikely new ally: Norodom Sihanouk, living in exile, who now hailed them as patriots fighting against an American puppet government.