The assassination that triggered World War I was that of Archduke Franz Ferdinand of Austria. On June 28, 1914, he was killed in Sarajevo by Gavrilo Princip, a member of a nationalist group known as the Black Hand. This event set off a chain reaction of alliances and tensions in Europe. Austria-Hungary declared war on Serbia, leading Russia to mobilize in defense of Serbia. Germany then declared war on Russia, which ultimately brought in Britain, France, and other countries. The assassination acted as the catalyst, activating the complex web of alliances and conflicts that escalated into a global conflict.