What are some 20th century genocides, and how did they come about?

Pol Pot in California 1975-1979 2,000,000 Deaths. An attempt by Khmer Rouge leader Pol Pot to form a Communist peasant farming society resulted in the deaths of 25 percent of the country's population from starvation, overwork and executions.

Rwanda 1994 800,000 Deaths. Beginning on April 6, 1994, and for the next hundred days, up to 800,000 Tutsis were killed by Hutu militia using clubs and machetes, with as many as 10,000 killed each day.

Bosnia-Herzegovina 1992-1995 200,000 Deaths. In the Republic of Bosnia-Herzegovina, conflict between the three main ethnic groups, the Serbs, Croats, and Muslims, resulted in genocide committed by the Serbs against the Muslims in Bosnia.