The event that led to the start of World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914. This assassination, carried out by Gavrilo Princip, a member of the nationalist group known as the Black Hand, set off a chain reaction of political alliances and military mobilizations throughout Europe. Following the assassination, Austria-Hungary issued an ultimatum to Serbia, leading to a declaration of war. This sparked a series of conflicts involving major powers, which were allied through treaty obligations, ultimately escalating into a global war that lasted from 1914 to 1918 and reshaped international relations forever.