The Rwandan genocide

Following the shooting down of the president's plane in April 1994, Hutu extremist militias and state forces kill Tutsi and moderate Hutu civilians, largely with machetes, coordinated in part through radio broadcasts. Estimates run from 500,000 to over a million. UN forces present were denied a mandate to intervene and were reduced rather than reinforced. It is the clearest case of preventable mass atrocity in the post-1945 order.