Austria-Hungary declared war on Serbia on July 28, 1914, marking the start of World War I.
The declaration followed the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, and his wife Sophie in Sarajevo on June 28. Austria-Hungary blamed Serbia for supporting forces it believed were linked to the assassination and issued an extremely demanding ultimatum in July.
Although Serbia accepted most of the ultimatum’s conditions, Austria-Hungary judged the response insufficient and declared war. The conflict initially appeared to be a limited Austro-Serbian war, but Russia mobilized in support of Serbia. Germany backed Austria-Hungary, while France and Britain became involved through their alliances and the German invasion of Belgium.
The date is sometimes confused with June 28, the date of the assassination, or August 4, when Britain declared war on Germany. July 28 identifies the first formal declaration of war in the chain of events that became the First World War.