World War II in Yugoslavia refers to World War II military operations that occurred on the territory of the then-Kingdom of Yugoslavia. Conflict in Yugoslavia began on 6 April 1941, when the Kingdom of Yugoslavia was swiftly conquered by Axis forces and partitioned between Germany, Italy, Hungary, Bulgaria and their client regimes. Shortly after Germany attacked the USSR on 22 June 1941, the communist-led republican Yugoslav Partisans, on orders from Moscow, launched a guerrilla liberation war fighting against the Axis forces and their locally established puppet regimes, including the Axis-allied Independent State of Croatia and the Government of National Salvation in the German-occupied territory of Serbia. This was dubbed the National Liberation War and Socialist Revolution in post-war Yugoslav historiography. Simultaneously, a multi-side civil war was waged between the Yugoslav communist Partisans, the Serbian royalist Chetniks, the Axis-allied Croatian Ustashas and Home Guard, Serbian Volunteer Corps and State Guard, Slovene Home Guard, as well as Nazi-allied Russian Protective Corps troops.
Both the Yugoslav Partisans and the Chetnik movement initially resisted the Axis rule.