The assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, on June 28, 1914, was the trigger that set off a chain of events leading to the outbreak of World War I.
The assassination was carried out by a Serbian nationalist named Gavrilo Princip 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 led to a diplomatic crisis between Austria-Hungary and Serbia, with Austria-Hungary issuing an ultimatum to Serbia that was seen as unacceptable.
When Serbia failed to comply with the ultimatum, Austria-Hungary declared war on Serbia on July 28, 1914.
Russia, as Serbia's ally, began to mobilize its army in support of Serbia, which prompted Germany, as Austria-Hungary's ally, to declare war on Russia on August 1, 1914.
France, as Russia's ally, then declared war on Germany on August 3, 1914, and Germany subsequently invaded Belgium, which brought Great Britain into the war on August 4, 1914.
Thus, the assassination of Archduke Franz Ferdinand is considered the immediate cause of World War I.