Created after WW I, from the kingdoms of Serbia and Montenegro and territories which had formerly belonged to the Turkish and Austro-Hungarian empires. Yugoslavia had been ruled since 1934 by the weak Prince Paul, regent for the young King Peter, whose father had been assassinated in that year. Although his sympathies lay with Britain, due to the presence of Italian military forces in neighbouring Albania and Greece Paul felt compelled to submit to an Axis Tripartite Pact with Italy and Germany on 25 March 1941. A popular uprising in Belgrade pushed out the puppet regime within three days, but on 6 April, in an operation named RETRIBUTION, Hitler invaded Yugoslavia. Although the main urban centres were occupied within two weeks, it was never pacified.
Initially the communists under Tito had to co-operate with the occupiers, as the non-aggression pact between Tito's political master Stalin and Hitler was still in force. BARBAROSSA was Tito's signal to open hostilities, although he had already taken to the hills before that. His forces numbered fewer than 10, 000, and competed with Col Draza Mihailovic's royalist all-Serb Chetniks, whom he eventually fought as viciously as the Germans. Following the April invasion, Hitler carved out of Yugoslavia the fascist state of Croatia (presided over by a local warlord, Pavelic, and policed by his brutal Ustashe).
Tito's mostly Serb partisans (a title, not merely a description) were initially regarded with suspicion by Mihailović and not welcomed by much of the population on whom the Ustashe, Italians, and Germans heaped brutal reprisals. In early 1942 the Partisans were forced out of Croatia, moving into the Bosnian countryside, and were restructured into brigades, in imitation of Stalin's bands of resistance fighters operating behind German lines. Care is needed when discussing military formations in Yugoslavia, for the number of personnel in Partisan ‘brigades’ and ‘divisions’ fluctuated wildly and did not necessarily correspond to units of similar designation operated by regular armies.
In 1943 Britain sent military missions to both Mihailović and Tito to ascertain which was the better bet in defeating the Germans, and thus worthy of military aid. The favourable reports of the senior British officer at Tito's HQ, Brig Fitzroy Maclean, plus accusations reaching London that Mihailović was collaborating with the Germans, caused the British to divert all support to Tito, and increase the quantity of aid substantially, a decision confirmed by the ‘big three’ at the Tehran Conference that November.
The accusations against Mihailović need to be treated carefully, because there is evidence that in mid-1943 Tito himself actually concluded a temporary truce with the Germans in order to defeat Mihailović's Chetniks (an understandable lure for the Germans, who were keen to see the elimination of at least one guerrilla band). German-Partisan hostilities were soon resumed, but Tito had established his force as the dominant resistance movement. In September 1943, Italy concluded an armistice with the Allies, which brought almost four divisions worth of men with their equipment over to the Partisans, and gave the British a base across the Adriatic from which to supply them more directly.
Gaining additional military support from Stalin and with the Red Army poised on Yugoslavia's eastern borders, Belgrade fell in October 1944, and by the end of the year the country was firmly under Partisan control. Operations in the north Balkans aimed at the annexation of Austrian and Italian territory brought Partisan forces into potential confrontation with the British occupying Trieste in April-May 1945, from which they withdrew. The Partisans were the only resistance movement to liberate their own country without the intervention of major foreign ground forces, but at a staggering cost of over a million Yugoslav lives, more by fratricidal conflict than at the hands of the Germans.
— Peter Caddick-Adams
