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.
Both the Yugoslav Partisans and the Chetnik movement initially resisted the Axis invasion. However, after 1941, Chetniks extensively and systematically collaborated with the Italian occupation forces until the Italian capitulation, and thereon also with German and Ustaše forces. The Axis mounted a series of offensives intended to destroy the Partisans, coming close to doing so in the Battles of Neretva and Sutjeska in the spring and summer of 1943.
