World War I began with the assassination of Archduke Franz Ferdinand of Austria-Hungary. He was killed on June 28, 1914, in Sarajevo, Bosnia, by a Serbian nationalist named Gavrilo Princip. This event triggered a chain of events that led to the outbreak of the war. Austria-Hungary declared war on Serbia, and a series of alliances meant that other countries were drawn into the conflict. Ultimately, World War I involved many of the world's major powers and resulted in the deaths of millions of people.