The primary cause of World War I was the complex web of alliances and rivalries that developed between the major European powers, combined with militarism, nationalism, and colonial competition. The assassination of Archduke Franz Ferdinand of Austria-Hungary in June 1914 acted as a catalyst, rapidly triggering a series of diplomatic crises that escalated tensions. The existing alliances divided Europe into opposing camps: the Allies and the Central Powers. Nationalist sentiments fueled tensions, while militarism led countries to prepare for conflict. These factors, intertwined, created an environment ripe for war, resulting in a devastating global conflict that reshaped the geopolitical landscape.