World War II in the Kingdom of Yugoslavia began on 6 April 1941, when the country was invaded and swiftly conquered by Axis forces and partitioned among 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 (NDH) 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 communist historiography. Simultaneously, a multi-side civil war was waged between the Yugoslav communist Partisans, the Serbian royalist Chetniks, the Axis-allied Croatian Ustaše and Home Guard, Serbian Volunteer Corps and State Guard, Slovene Home Guard, as well as Nazi-allied Russian Protective Corps troops.

The Yugoslav Partisans, or the National Liberation Army, officially the National Liberation Army and Partisan Detachments of Yugoslavia, was the communist-led anti-fascist resistance to the Axis powers in occupied Yugoslavia during World War II. Led by Josip Broz Tito, the Partisans are considered to be Europe's most effective anti-Axis resistance movement during World War II.