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 was Gavrilo Princip, 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 was the culmination of a series of tensions between Austria-Hungary and Serbia. Austria-Hungary had long been suspicious of Serbia's intentions and was concerned about the growing influence of Serbian nationalism in the region. Serbia, for its part, resented Austria-Hungary's control of Bosnia-Herzegovina and was eager to expand its territory and unite all South Slavic peoples into a single state.
The assassination of Archduke Franz Ferdinand provided Austria-Hungary with the pretext it needed to take action against Serbia. On July 28, 1914, Austria-Hungary declared war on Serbia, marking the beginning of World War I.