Assassination of Archduke Franz Ferdinand

In 1913, Emperor Franz Joseph commanded Archduke Franz Ferdinand, heir presumptive to the Austro-Hungarian throne, to visit the military exercises in Bosnia in June of 1914. Franz Ferdinand and his wife, Sophie, Duchess of Hohenberg were assassinated on 28 June 1914 in Sarajevo by Gavrilo Princip. Princip was one of a group of six assassins (one Bosniak and five Serbs) coordinated by Danilo Ilić, a Bosnian Serb and a member of the Black Hand secret society.
The purpose of assassination was to break off Austria-Hungary’s South Slav provinces so that they can be combined into Yugoslavia.
The assassination led directly to World War I. Austria-Hungary issued an ultimatum to the Kingdom of Serbia after it happened. However, a part of them was rejected by Serbia since Russia promised military support for Serbia. Then, Austria-Hungary declared war on Serbia because Germany promised military support.
Under the Secret Treaty of 1892, Russia and France were obliged to mobilize their armies if any of the Triple Alliance mobilized. Russia ordered general mobilization. With the Russia's general mobilization, Austria-Hungary and Germany also Austro-Hungarian and German mobilizations. Soon all the Great Powers mobilize their armies and gone to war. As the assassination of Franz and Sophia was the trigger of starting World War I, it is important historically especially for European countries.