World War I was a global war centered in Europe that began in 1914 and lasted until 1918. It involved over 30 countries that were split into two military alliances: the Allies and the Central Powers. The war began after the assassination of Archduke Franz Ferdinand of Austria by Serbian nationalists. This caused a chain reaction of military alliances that led to the outbreak of war in Europe. The United States joined the Allies in 1917 after years of neutrality. The war ended with the defeat of the Central Powers and the signing of an armistice in 1918.