There were contingencies for internal disputes with the outbreak of World War II extending to Yugoslavia in AD 1941. The occupation of Yugoslavia by the Allied forces led to the formation of different militia groups with different goals, ideologies, and political backgrounds. Such armed groups attempted to take military measures and fought not only against occupation but also for post-war political fate and future. In this way, many changes and fluctuations in their policies and positions emerged. Against this background, the authors seek to shed light on the aims of militia groups and their approach to changes in Yugoslavia in World War II as well as the factors effective in the success or failure of each of the involved groups in achieving their goals. The findings of this descriptive-analytical study show that eventually the leftist partisan movement succeeded to defeat its opponents and take over the affairs of the country by gaining greater domestic and foreign support. Political and military changes of this era of Yugoslavia's history determined the post-war political future of the country and became a foundation for developments in the following decades.