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 and a member of the Black Hand, a secret society dedicated to unifying all South Slavic peoples into a single state.
The assassination set off a chain of events that led to the outbreak of World War I. Although Austria-Hungary had long been concerned about the threat of Serbian nationalism, the assassination provided a catalyst for action. The Austrian government issued an ultimatum to Serbia, demanding that it take action to suppress the Black Hand and punish those responsible for the assassination. Serbia agreed to most of the demands, but some were deemed unacceptable. As a result, Austria-Hungary declared war on Serbia on July 28, 1914.
Russia, as Serbia's ally, began mobilizing its army in support of Serbia. Germany, as Austria-Hungary's ally, responded by declaring war on Russia on August 1, 1914, and France, as Russia's ally, on August 3, 1914. This marked the beginning of World War I.