Titos partisans are formed in 1941 and begin guerilla operations against the axis.
Until 1945 those operations included railway bombings, ambushes, assasinations kindapings, espionage, helping shot down ally pilots, enciting further insurection, raids and general unpleasantry to the ocupiers. In the later years that incuded a freed area with it's own government, it's own airforce and a tank battalion (the famed french resistance was a disorganised rabble in comparison). They tied up several German divisions that could have been efectively used in Russia.
27th April is still a national holliday in the former republics even after the Yugoslav dissolution.
