World War I
Triggered by the assassination in Sarajevo, the European powers fought in two blocs. Four empires collapsed.

John Warwick Brooke · Public domain · Source
In the late 19th century the great powers, competing for colonies, split into the Triple Alliance of Germany, Austria-Hungary and Italy and the Triple Entente of Britain, France and Russia. In the Balkans, where Pan-Germanism clashed with Pan-Slavism, the region became known as the "powder keg of Europe". When the Austrian heir and his wife were assassinated in Sarajevo by a young Serbian nationalist on 28 June 1914, Austria declared war on Serbia on 28 July and the powers were drawn in one after another through their alliances.
Germany invaded France through Belgium but was halted at the Marne, and the Western Front settled into trench warfare. Machine guns, poison gas, tanks and aircraft appeared in a total war that mobilised whole nations. Germany’s unrestricted submarine warfare brought the United States into the war in 1917, and revolutionary Russia left it with the Treaty of Brest-Litovsk in March 1918.
The war ended when Germany signed the armistice on 11 November 1918. More than fifteen million soldiers and civilians are estimated to have died. Four empires — the German, Austro-Hungarian, Russian and Ottoman — collapsed.
At the Paris Peace Conference of 1919 the Treaty of Versailles was signed and the League of Nations founded (the Versailles system). President Wilson’s principle of national self-determination was applied only to the territories of the defeated powers, but it gave hope to colonised peoples and influenced Korea’s March First Movement and China’s May Fourth Movement.