Gavrilo Princip, a Bosnian Serb nationalist and member of the Black Hand, a secret society aiming for the unification of all South Slavic peoples into a single state, carried out the assassination. Princip shot and killed both Archduke Franz Ferdinand and his wife, Sophie, Duchess of Hohenberg, during an official visit to Sarajevo.
This event precipitated a chain of diplomatic crises and military mobilizations among the major European powers, which were already divided into two rival alliances: the Triple Alliance (Germany, Austria-Hungary, and Italy) and the Triple Entente (Britain, France, and Russia).
Austria-Hungary, with Germany's backing, issued an ultimatum to Serbia on July 23, 1914, demanding a full investigation into the assassination and punishment of those responsible. Serbia accepted most of the demands but rejected some that would compromise its sovereignty. This triggered Austria-Hungary's declaration of war against Serbia on July 28, 1914.
Russia, in support of its Slavic ally Serbia, began mobilizing its military, prompting Germany to issue an ultimatum demanding Russia halt its mobilization within 12 hours. When Russia did not comply, Germany declared war on Russia on August 1, 1914. Germany also declared war on France, Russia's ally, on August 3, 1914, after learning of French intentions to mobilize its military.
As a result of these actions, a complex web of alliances and mutual defense pacts led to the involvement of other European countries, marking the official start of World War I.