World War I started on 28 July 1914 with Austria-Hungary's declaration of war against Serbia, motivated with the assassination of Archduke Franz Ferdinand of Austria, the heir to the Austro-Hungarian throne, in Sarajewo by a Serbian student. This set off a chain reaction with the Russian Empire supporting Serbia, France supporting the Russian Empire and German Empire supporting Austria-Hungary. The United Kingdom declared war against the German Empire following the violation of Belgian neutrality by Germany in the starting phase of the war. More European countries joined the war later and the USA entered the war in 1917 after the German submarines sank seven U.S. merchant ships.
After the German march on Paris was brought to a halt, the Western Front settled into a static battle of attrition with a trench line that changed little until 1917. The battles on the Eastern Front were fought with varying success until the Russian Empire collapsed in 1917 and the new communist government signed a piece treaty with Germany. Nevertheless the war ended with a defeat of Germany and Austria-Hungary. Germany signed an armistice on 11 November 1918 and was later forced to sign the Treaty of Versailles in which it took responsibility for the war and agreed to wide-reaching concessions as well as high reparation payments.