The chief event that sparked the beginning of World War I was the assassination of Archduke Franz Ferdinand of Austria on June 28, 1914, in Sarajevo. His assassination by Gavrilo Princip, a member of a nationalist group seeking the end of Austro-Hungarian rule over Bosnia, set off a chain reaction of political alliances and militaristic tensions. Following the assassination, Austria-Hungary issued an ultimatum to Serbia, leading to declarations of war due to a web of treaties among European powers. This rising tension ultimately escalated into a full-scale global conflict.