Eh, it's a bit more complicated than that (like most things). When the Vietnamese removed the genocidal Khmer Rouge, the United States opposed the action (viewing it as Vietnamese expansionism). The Khmer Rouge joined with other factions to form a coalition that fought the Vietnamese and the new government, and the U.S. supported this coalition.[1][2]
I wouldn't call them powerless; they were waging a guerilla war in the countryside into the 1990's, and were able to retain their seat in the United Nations for some time (with the help of Western governments).
[1] http://www.upi.com/Archives/1983/09/22/Reagan-to-meet-Cambod... [2] http://articles.chicagotribune.com/1990-07-18/news/900228054...