Oh yes, we did! When Vietnam invaded Cambodia because it could no longer tolerate the atrocities across their border by the psychotic Pol Pot, the United States backed the Pol Pot regime!
I guess our leaders were still sore that those funny little guys in their black pajamas kicked our asses out of Vietnam. So sore they were that they decided to support the murderous Pol Pot regime against Vietnam.
Vietnam toppled Pol Pot and pulled its troops out, letting the progressive Cambodians get their house in order.
In 1978, Vietnam invaded Cambodia to oust the dictatorship of Pol Pot. The invasion was a response to two years of border incursions by Pol Pot's forces.
These incursions, which began on May 1, 1975, the very day after the American evacuation, had resulted in the deaths of 30,000 civilians, the destruction of border villages and the abandonment of vast tracts of agricultural land.
Within a fortnight of the invasion, the Khmer Rouge had been driven out into Thailand. In its place, the Vietnamese installed a government led by Cambodian communists who had opposed Pol Pot.
The liberation of Cambodia set off a chain of events which led to the isolation of Vietnam. Pol Pot had been China's protege. By the late seventies, China and the US had virtually become allies in international affairs.
Together they ensured that Thailand gave sanctuary to the remains of the Khmer Rouge. The Khmer Rouge was rearmed and rebuilt into a guerrilla army, an ever-present threat to the population.
Despite the proven massacre of more than a million Cambodians, the international community continued to recognise the Khmer Rouge as the legitimate government of Cambodia, keeping its seat at the UN.
http://pilger.carlton.com/vietnam/war_invasion