The assassination of Archduke Franz Ferdinand on June 28, 1914, is marked as the event that triggered World War I. The heir to the Austro-Hungarian throne, his murder by nationalist Gavrilo Princip set off a chain reaction of alliances and mobilizations among European powers. The assassination heightened existing tensions and rivalries, leading to Austria-Hungary's declaration of war against Serbia, which in turn activated a web of treaties involving multiple countries. This catastrophic escalation ultimately resulted in a global conflict that caused unprecedented loss of life and laid the groundwork for significant political changes and events in the 20th century.