Primarily a guerrilla force at its inception, the Partisans developed into a large fighting force engaging in conventional warfare later in the war, numbering around , in late and organized in four field armies and 52 divisions.
The main stated objectives of the Partisans were the liberation of Yugoslav lands from occupying forces and the creation of a federal, multi-ethnic socialist state in Yugoslavia. The Partisans were organized on the initiative of Tito following the Axis invasion of Yugoslavia in April , and began an active guerrilla campaign against occupying forces after Germany invaded the Soviet Union in June.
The Axis mounted a series of offensives in response but failed to completely destroy the highly mobile Partisans and their leadership. By the end of the war, the Partisans had gained control of the entire country as well as Trieste and Carinthia.
After the war, the Partisans were reorganized into the regular armed force of the newly established Federal People's Republic of Yugoslavia. Until British supplies began to arrive in appreciable quantities in , the occupiers were the only source of arms.
The objectives of the rival resistance movement, the Chetniks , were the retention of the Yugoslav monarchy , ensuring the safety of ethnic Serb populations, [ 20 ] [ 21 ] and the establishment of a Greater Serbia [ 22 ] through the ethnic cleansing of non-Serbs from territories they considered rightfully and historically Serbian.
