How did the assassination of Archduke Franz Ferdinand lead to World War I?

The assassination of Archduke Franz Ferdinand on June 28, 1914, served as the immediate catalyst for World War I. The archduke was killed in Sarajevo by Gavrilo Princip, a Bosnian Serb nationalist associated with the Black Hand, a group seeking to end Austro-Hungarian rule in the Balkans and unite South Slavs under Serbia.12

For Austria-Hungary, the event provided a long-sought opportunity to eliminate the perceived threat of Serbian nationalism. With the backing of Germany, the Austro-Hungarian government issued a deliberately provocative ultimatum to Serbia on July 23, 1914. When Serbia’s response was deemed insufficient, Austria-Hungary severed diplomatic ties and declared war on July 28.3452

This local conflict rapidly escalated into a continental war due to the existing system of alliances. Russia mobilized to support Serbia, while France honored its alliance with Russia. Germany, seeking to strike before its rivals could fully mobilize, invaded neutral Belgium, prompting Britain to enter the war on August 4, 1914.5