The reprisals for collaboration with the Nazis were particularly harsh in Yugoslavia, because collaborators were also on the losing side of a "de facto" civil war fought on the Yugoslav territory during World War II. The Communists executed many Ustashe, as well as their collaborators, particularly in a series of atrocities referred to as the Bleiburg tragedy. After the war, the UDBA, Yugoslavia's secret police, was sent overseas to find and eliminate several former Ustashe who fled the country, including the leader of the Ustashe and their pro-Nazi government, Ante Pavelić.