The event that triggered 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 the nationalist group known as the Black Hand. This assassination set off a chain reaction of alliances and tensions among European powers. Austria-Hungary issued an ultimatum to Serbia, which led to declarations of war as countries mobilized in support of their allies. The intricate web of alliances, militarism, and nationalist sentiments contributed to the swift escalation into a full-scale war.