The event that precipitated the start of World War I was the assassination of Archduke Franz Ferdinand of Austria on June 28, 1914. This event triggered a series of diplomatic crises and alliances in Europe. Austria-Hungary declared war on Serbia, activating a network of alliances and leading to a full-scale war. The complex system of alliances among major powers, including Germany, Russia, France, and Britain, shifted the conflict from a regional issue into a global war, ultimately resulting in widespread devastation.