This paper elaborates on the causes that led to the collapse of once powerful Austro-Hungarian Monarchy and the consequences that emerged in the creation of new states, one of them being the state of State of Slovenes, Croats and Serbs. Since the beginning of the 20th century, the Yugoslav idea has gained more importance and became “more popular” among Croatian politicians, primarily because of the difficult conditions they lived in and the Hungarianization and Germanization that was becoming stronger and stronger. The author is presenting key factors, form 1903 until 1918, which are of a crucial importance for this subject. However, the focus is mainly on the period of First World War, that is, on the time span between years 1914 and 1918. During this period, all the causal links that led to the unfavourable circumstances in which the Austro-Hungarian Monarchy had found itself can be addressed. As the Monarchy was getting in a more difficult state with every passing day, the South Slav movement was getting stronger. The Yugoslav Committee, which was supposed to represent the South Slavs of the Monarchy, failed in its plan on the European scene, the politicians in Croatian countries were uniting and working to create a new state. Originally, on October 19, after the leader of the leading Croatian-Serbian coalition Svetozar Pribićević agreed, the National Council of Slovenians, Croats and Serbs was established as the supreme body for making all decisions. At the session of the Parliament on October 29, they finally decided to break all strings between Croatia, Slavonia and Dalmatia with Hungary and Austria, and the new State of SHS was founded. Due to the extremely difficult situation and unfavourable international position, this newly established SHS State had only a provisional character, and, under the influence of Pribicevic, united with the Kingdom of Serbia into the Kingdom of 113 Serbs, Croats and Slovenes, where Croatia lost its millennial historical continuity and statehood.