The event that triggered the onset of World War I was the assassination of Archduke Franz Ferdinand of Austria on June 28, 1914. This pivotal incident in Sarajevo led Austria-Hungary to issue an ultimatum to Serbia, which in turn prompted a series of alliances and mobilizations among the European powers. The complex web of treaties and mutual defense agreements resulted in a rapid escalation of tensions, culminating in declarations of war by multiple nations. The assassination acted as a catalyst for pent-up rivalries and nationalist sentiments, ultimately plunging Europe into a devastating conflict that would last four years and reshape global politics.