On June 28, 1914, Archduke Franz Ferdinand of Austria, the heir to the Austro-Hungarian throne, was assassinated in Sarajevo, the provincial capital of Bosnia-Herzegovina (formally annexed by Austria-Hungary in 1908). The assassin was Gavrilo Princip, a Bosnian Serb and member of the Black Hand, a secret society dedicated to the unification of all South Slavic peoples into a single state.
Austria-Hungary's Ultimatum to Serbia
Austria-Hungary, seeing the assassination as a direct threat to its sovereignty and territorial integrity, issued an ultimatum to Serbia on July 23, 1914, demanding that it accept a series of conditions, including the suppression of anti-Austrian propaganda, the arrest and punishment of those involved in the assassination, and the allowance of Austrian officials to participate in the investigation. Serbia accepted most of the demands, but rejected those that would have compromised its sovereignty.
Russian Mobilization and Germany's Declaration of War
Russia, as Serbia's ally and protector, mobilized its forces in response to the Austrian ultimatum, prompting Germany, which had a mutual defense alliance with Austria-Hungary, to declare war on Russia on August 1, 1914. France, as Russia's ally, declared war on Germany the following day. Germany then invaded Belgium, a neutral country, leading Britain, which had a treaty obligation to protect Belgian neutrality, to declare war on Germany on August 4, 1914.
Domino Effect of Alliances
The conflict quickly escalated into a global war involving most of the major powers of the time. The system of alliances between European nations, known as the "Triple Alliance" (Germany, Austria-Hungary, and Italy) and the "Triple Entente" (Britain, France, and Russia), drew more countries into the war as each alliance supported its partners. By the end of 1914, the war had spread to include Germany, Austria-Hungary, the Ottoman Empire, Bulgaria, and Montenegro on one side, and Britain, France, Russia, Italy, Belgium, Serbia, and Japan on the other.
In summary, the assassination of Archduke Franz Ferdinand in Sarajevo was the proximate cause of World War I. The event set in motion a chain of diplomatic crises and military mobilizations that ultimately led to the outbreak of war between the major European powers.