How many countries were involved in World War I and When did the United States join World War I?

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.