Yugoslavia in World War II The involvement of the Yugoslavia in World War II began with the invasion of the Kingdom of Yugoslavia by the Axis powers in 1941 . In 1943 , Democratic Federal Yugoslavia was proclaimed by the Partisan resistance . In 1944 , the king recognised it as the legitimate government , but in November 1945 the monarchy was abolished . Yugoslavia was renamed the Federal People ' s Republic of Yugoslavia in 1946 , when a communist government was established . Invasion of Yugoslavia . At 5 : 12 am on 6 April 1941 , German , Italian and Hungarian forces attacked Yugoslavia . The German Air Force ( " Luftwaffe " ) bombed Belgrade and other major Yugoslav cities . On 17 April , representatives of Yugoslavia ' s various regions signed an armistice with Germany in Belgrade , ending 11 days of resistance against the invading German Army ( " Wehrmacht Heer " ) . More than 300 , 000 Yugoslav officers and soldiers were taken prisoner . The Axis Powers occupied Yugoslavia and split it up . The Independent State of Croatia was established as a Nazi satellite state , ruled by the fascist militia known as the Ustaše that came into existence in 1929 , but was relatively limited in its activities until 1941 . German troops occupied Bosnia and Herzegovina as well as part of Serbia and Slovenia , while other parts of the country were occupied by Bulgaria , Hungary , and Italy . From 1941 – 45 , the Croatian Ustaše regime murdered around 500 , 000 people , 250 , 000 were expelled , and another 200 , 000 were forced to convert to Catholicism ; the victims were predominantly Serbs but included 37 , 000 Jews . National Liberation War . From the start , the Yugoslav resistance forces consisted of two factions : the communist - led Yugoslav Partisans and the royalist Chetniks , with the former receiving Allied recognition only at the Tehran conference ( 1943 ) . The heavily pro - Serbian Chetniks were led by Draža Mihajlović , while the pan - Yugoslav oriented Partisans were led by Josip Broz Tito . The Partisans initiated a guerrilla campaign that developed into the largest resistance army in occupied Western and Central Europe . The Chetniks were initially supported by the exiled royal government and the Allies , but they soon focused increasingly on combating the Partisans rather than the occupying Axis forces . By the end of the war , the Chetnik movement transformed into a collaborationist Serb nationalist militia completely dependent on Axis supplies . The highly mobile Partisans , however , carried on their guerrilla warfare with great success . Most notable of the victories against the occupying forces were the battles of Neretva and Sutjeska . On 25 November 1942 , the Anti - Fascist Council of National Liberation of Yugoslavia was convened in Bihać , modern day Bosnia and Herzegovina . The council reconvened on 29 November 1943 , in Jajce , also in Bosnia and Herzegovina , and established the basis for post - war organization of the country , establishing a federation ( this date was celebrated as Republic Day after the war ) . The Yugoslav Partisans were able to expel the Axis from Serbia in 1944 and the rest of Yugoslavia in 1945 . The Red Army provided limited assistance with the liberation of Belgrade and withdrew after the war was over . In May 1945 , the Partisans met with Allied forces outside former Yugoslav borders , after also taking over Trieste and parts of the southern Austrian provinces of Styria and Carinthia . However , the Partisans withdrew from Trieste in June of the same year . Western attempts to reunite the Partisans , who denied the supremacy of the old government of the Kingdom of Yugoslavia , and the émigrés loyal to the king led to the Tito - Šubašić Agreement in June 1944 ; however , Marshal Josip Broz Tito was seen as a national hero by the citizens and was elected by referendum to lead the new independent communist state , starting as a prime minister . The official Yugoslav post - war estimate of victims in Yugoslavia during World War II is 1 , 704 , 000 . Subsequent data gathering in the 1980s by historians Vladimir Žerjavić and Bogoljub Kočović showed that the actual number of dead was about 1 million .