Pol Pot's weapons came from China and North Vietnam. Pol Pot sent Cambodian rice to China in exchange for weapons, knowing full well that not enough food would be left for the cambodian population.
Coincidentally, Pol Pot was politicised/radicalised in France by the French communist party, received ideological training in Moscow, Beijing and former Yugoslavia. I imagine that at least some of those who organised Pol Pot's take-over are still alive in in France, the former Soviet Union, China and former Yugoslavia, and could be brought to justice.
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).
Coincidentally, Pol Pot was politicised/radicalised in France by the French communist party, received ideological training in Moscow, Beijing and former Yugoslavia. I imagine that at least some of those who organised Pol Pot's take-over are still alive in in France, the former Soviet Union, China and former Yugoslavia, and could be brought to justice.