Archduke Francis Ferdinand's Assasination started World War I

Franz Ferdinand (18 December 1863 – 28 June 1914) was an Archduke of Austria-Este, Austro-Hungarian and Royal Prince of Hungary and of Bohemia, and from 1889 until his death, heir presumptive to the Austro-Hungarian throne. His assassination in Sarajevo precipitated Austria-Hungary's declaration of war against Serbia. This caused countries allied with Austria-Hungary (the Triple Alliance) and countries allied with Serbia (the Triple Entente Powers) to declare war on each other, starting World War I. from Wikipedia