The tragic event that led to the start of World War I was the assassination of Archduke Franz Ferdinand of Austria on June 28, 1914. He was killed in Sarajevo by Gavrilo Princip, a member of a nationalist group seeking independence for Serbia from Austro-Hungarian rule. This assassination set off a chain reaction of events, leading to Austria-Hungary's declaration of war against Serbia. The complex system of alliances among European powers escalated the conflict, involving major nations in support of either side, which ultimately culminated in a full-scale global war. This event marked the beginning of a devastating conflict with lasting repercussions.