On June 28, 1914, Archduke Franz Ferdinand of Austria, the heir to the throne of Austria-Hungary, was assassinated in Sarajevo, the capital of the province of Bosnia-Herzegovina (formally annexed by Austria-Hungary in 1908). The assassin, Gavrilo Princip, was a Bosnian Serb nationalist who was a member of the Black Hand, a secret society dedicated to the unification of all South Slavic peoples into a single state.
The assassination of Archduke Franz Ferdinand set off a chain of events that led to the outbreak of World War I. The Austrian government, with the backing of Germany, issued an ultimatum to Serbia, demanding that it punish those responsible for the assassination and take steps to prevent similar incidents in the future. Serbia agreed to most of the demands, but refused to allow Austrian officials to participate in the investigation of the assassination. This led Austria-Hungary to declare war on Serbia on July 28, 1914.
Russia, which had a mutual defense pact with Serbia, began mobilizing its army in response to Austria-Hungary's declaration of war. Germany, which had a military alliance with Austria-Hungary, responded by declaring war on Russia on August 1, 1914, and on France, Russia's ally, on August 3, 1914. Thus began World War I.