Before 1918, eleven officially recognized nationalities lived in Austria-Hungary. In Styria, these were the German Styrians and the Slovenes, with the latter making up about a third of the population. For decades, there had been intense conflicts between them, centered around the Slovenian claim for the realization of the theoretically promised equality. With the assassination of the heir to the throne, Franz Ferdinand, in Sarajevo and the outbreak of World War I, the latent hatred against non-German nations in Austria-Hungary became virulent. In the summer of 1914, a true wave of denunciations swept over the Styrian Slovenes. They were accused of secret or even open collaboration with the Serbian enemy and were arrested by the hundreds. Utilizing a wide range of source material for the first time, this analysis examines the national conflicts between the two groups living in the Duchy of Styria before 1914, the German Styrians and the Slovenes, as well as their escalation at the outbreak of the world war in the summer of 1914. These conflicts were a significant precondition for the internal disintegration of the monarchy in the autumn of 1918 and the division of Styria.