The main cause of World War I was the complex web of alliances and rivalries among European powers, exacerbated by nationalism, militarism, and imperial ambitions. The immediate catalyst was the assassination of Archduke Franz Ferdinand of Austria-Hungary in June 1914, which triggered a series of diplomatic conflicts. The Alliance System split Europe into two main factions: the Allies, including France, Russia, and the United Kingdom, and the Central Powers, led by Germany, Austria-Hungary, and later the Ottoman Empire. This underlying tension, combined with the desire for territorial expansion, ultimately escalated into a full-scale war, resulting in unprecedented casualties and geopolitical changes.