The spark that ignited World War I was the assassination of Archduke Franz Ferdinand. His wife Sophie was also executed by the assassin.
The assassination took place on June 28, 1914, and set off a chain reaction throughout Europe. Archduke Franz Ferdinand was heir apparent to the throne of the Austro-Hungarian Empire. The southern Balkan regions of the Empire were predominantly Slav and home to Serbian nationalists who sought independence.
Initially, the Serbian nationalists attempted to throw a bomb at the open car in which the Archduke and his wife were traveling. However, the person who threw the bomb missed the target, and it rolled off the back of the vehicle, injuring bystanders instead. Later, as the couple was visiting someone wounded in the initial attempt, Gavrilo Princip, another Serbian nationalist, seized the opportunity and shot Franz Ferdinand and Sophie at close range. He attempted to turn the gun on himself, but bystanders prevented him from doing so.
Immediately, Austria-Hungary pointed the finger at the Serbian government for the attacks. Russia was an ally of the Serbs. In turn, France and Britain had an alliance with Russia. The divide between Serbia, Russia, France and Britain on one side and Germany and Austria-Hungary on the other soon gave way to World War I.