As WWII began, France fell, as did Yugoslavia's hopes of French aid, in 1940.

    In 1941, Hitler's army attacked Yugoslavia and on April 17, the government fled and those who remained surrendered. During the war, many different resistance groups were formed, and Tito's Partisan Communist group came to the forefront.

    Initially, Serb Chetnik forces were leading the way with Western support, but as resistance to Germanization and other tactics of the Axis powers continued, the power spectrum shifted. In 1941, the Chetniks and Partisans fought jointly against the Axis powers, but as ultimatums concerning reprisals by German forces were handed down, the Chetnik forces halted actions, while the Partisans pushed on. This led to the Chetniks fighting the Partisan forces, and even began siding with Germany and Italy to make efforts to prevent Communists from coming into power.

    The Partisans were encircled in 1943, but they fought their way out and as Italy was defeated in 1943, they began to receive supplies from Allied troops in Italy.

    In September of 1944, Stalin's army reached the Yugoslav boarder. Tito met with Stalin and they agreed that the army would enter the nation, but would leave as it became secured. After the Red Army passed through, it was simply a matter of time for the Partisans to clean house and destroy the remnants of other resistance groups.

    As WWII came to an end, Yugoslavia had the second highest mortality numbers (1st was Poland) - 11 percent of its prewar populous. This figure reached about 1.7 million, with one million of those having been killed by other Yugoslavs. source