 On 6 April 1941 Yugoslavia was invaded from all sides by the Axis powers , led by Nazi Germany . The Royal Yugoslav Army ( Serbo @-@ Croatian : Vojska Kraljevine Jugoslavije or VKJ ) capitulated on 17 April , and the Germans , Italians and Hungarians dismembered the country . A fascist puppet state , the Independent State of Croatia ( Serbo @-@ Croatian : Nezavisna Država Hrvatska or NDH , including almost all of modern @-@ day Croatia , all of modern @-@ day Bosnia @-@ Herzegovina and parts of modern @-@ day Serbia ) was proclaimed on 10 April . The NDH was an " Italian @-@ German quasi @-@ protectorate " , controlled by the Croatian nationalist Ustaše movement under Ante Pavelić . One of the NDH 's policies was to eliminate the state 's ethnic Serb population with mass killings , expulsions and forced assimilation . The first Drvar Serbs were killed on 18 June 1941 . The atrocities accelerated the formation of two large resistance movements in occupied Yugoslavia . Royalists and Serbian nationalists led by VKJ Colonel Draža Mihailović founded the Ravna Gora Movement , whose members were known as Chetniks . The KPJ , led by Josip Broz Tito , decided in Belgrade on 4 July to launch a nationwide armed uprising and the members of the KPJ @-@ led forces became known as Partisans . 
