History of Asia

What group killed nearly 2 million Cambodians in the vietnam war?

There is a mistake in your question. The Vietnam War did not involve Cambodia, and nearly 2 million Cambodians were killed during the Cambodian Genocide, which was carried out by the Khmer Rouge regime led by Pol Pot. The Khmer Rouge was a communist party in Cambodia that ruled the country from 1975 to 1979.