Mutual defense alliances between European countries meant that the assassination of Archduke Franz Ferdinand drew more countries into the conflict. Rising nationalism, imperialism, and militarism increased tensions, while disputes over territory in Africa and Asia were exacerbated by competition between colonial powers. The immediate trigger was the assassination of Archduke Franz Ferdinand by a Serbian nationalist, which caused Austria-Hungary to declare war on Serbia and activated the system of alliances that pulled other European countries into the growing conflict.