World War I was an armed conflict fought from 1914 to 1918 between the Allied Powers (including Great Britain, France, Russia, and the United States) and the Central Powers (Germany, Austria-Hungary, and Turkey).  Colonial rivalries, territorial ambitions, and shifting alliances were all factors that contributed to the outbreak of this conflict.

Allied armies halted early Central military successes, and a four-year stalemate of trench warfare followed.  In 1917, Germany and its allies succeeded in knocking Russia out of the war, but the arrival of U.S. forces in 1918 ultimately turned the tide against the Germans.

After the Allied victory, the political map of Europe changed drastically.  Monarchies in Germany, Austria-Hungary, and Russia were overthrown, a communist government took power in Moscow and established the Soviet Union, and new nations were carved out of eastern Europe.  Importantly, the Treaty of Versailles, signed by Germany and the Allies, imposed on Germany harsh terms that caused economic instability and created fertile ground for the rise of Adolf Hitler.