The assassination of Archduke Franz Ferdinand of Austria, heir to the Austro-Hungarian throne, on June 28, 1914, in Sarajevo, the provincial capital of Bosnia-Herzegovina (formally annexed by Austria-Hungary in 1908), is widely regarded as the immediate trigger of World War I.
Background
The assassination was carried out by a Bosnian Serb named Gavrilo Princip, a member of the nationalist organization Young Bosnia. Princip and his co-conspirators believed that Austria-Hungary's annexation of Bosnia-Herzegovina had violated the rights of the South Slavic population of the region and that the only way to achieve independence for Bosnia and Herzegovina was through armed resistance.
Austria-Hungary's Reaction
Austria-Hungary's government was outraged by the assassination and saw it as an opportunity to crush the Serbian nationalist movement once and for all. With the backing of Germany, Austria-Hungary issued an ultimatum to Serbia on July 23, 1914, demanding that it punish those responsible for the assassination, suppress anti-Austrian propaganda, and allow Austrian police officers to investigate the crime in Serbia.
Serbia's Response
Serbia accepted most of the demands but rejected the one that would have allowed Austrian police officers to operate on Serbian soil. Austria-Hungary considered this a rejection of the ultimatum and declared war on Serbia on July 28, 1914.
The Chain of Events
Russia, as Serbia's main ally, began to mobilize its military in response to Austria-Hungary's declaration of war. Germany, which had promised to support Austria-Hungary in the event of war, responded by declaring war on Russia on August 1, 1914, and on France, Russia's ally, on August 3, 1914.
World War I Begins
The German invasion of Belgium on August 4, 1914, in violation of Belgian neutrality, brought Great Britain into the war on the side of France and Russia. Thus, the assassination of Archduke Franz Ferdinand triggered a chain of events that led to the outbreak of World War I, a conflict that would ultimately claim the lives of millions of people.