The Ustaše, also known by anglicised versions Ustasha or Ustashe, was a Croatian, fascist and ultranationalist organization active, as one organization, between 1929 and 1945, formally known as the Ustaša – Croatian Revolutionary Movement. Since the inception and before the Second World War, its members engaged in a series of terrorist activities against the Kingdom of Yugoslavia, including collaborating with IMRO to assassinate King Alexander I of Yugoslavia in 1934. After the invasion of Yugoslavia by Axis forces in April 1941, the Ustaše were handed power by the occupying forces and formed the Independent state of Croatia, an Axis puppet state, and immediately went on to perpetrate the Holocaust and genocide against its Jewish, Serb and Roma populations. During the World War II in Yugoslavia, together with German and Italian occupying forces, Ustaše are responsible for killing of hundreds of thousands of Serbs, Jews, Roma, as well as Bosniak Muslim and Croat political dissidents.

The Independent State of Croatia was a World War II-era puppet state of Nazi Germany and Fascist Italy. It was established in parts of occupied Yugoslavia on 10 April 1941, after the invasion by the Axis powers. Its territory consisted of most of modern-day Croatia and Bosnia and Herzegovina, as well as some parts of modern-day Serbia and Slovenia, but also excluded many Croat-populated areas in Dalmatia, Istria, and Međimurje regions.