The event that sparked the beginning of World War I in 1914 was the assassination of Archduke Franz Ferdinand of Austria-Hungary. On June 28, 1914, he was killed in Sarajevo by Gavrilo Princip, a member of the nationalist group known as the Black Hand. This assassination set off a chain reaction of alliances and conflicts among European powers, leading to widespread warfare. Within weeks, the conflict escalated into a full-scale world war, significantly altering international relations and resulting in millions of casualties.