World War I lasted from 1914 to 1918. It began as a war between two European countries, Austria-Hungary and Serbia.
The underlying causes of the conflict were rooted in European political and economic policies after 1871. Eventually, the war involved thirty-two countries.
Twenty-eight of these countries were known as the Allies, and included Great Britain, France, Russia, and Italy. The other side, known as the Central Powers, included Germany, Austria-Hungary, the Ottoman Empire (present-day Turkey), and Bulgaria.
The United States joined the war in 1917 on the side of the Allies.