World War I lasted from 1914 until 1918. The two main belligerents in this conflict were Germany and the Austro-Hungarian Empire. World War II lasted from 1939 until 1945. The United States became involved in this conflict in 1941. The peace made after World War I did not last. There were a number of reasons for this, but one of the most significant was the refusal of the United States to join the League of Nations. Another was the insistence that Germany be financially responsible for reparations to France. The peace the was created after World War II included United States membership in the United Nations. This was an essential element preventing the outbreak of a third world war. Secondly, the United States invented billions of dollars in helping to rebuild European nations after the war. This was a key component to European prosperity after World War II.