While the ratification of the Treaty of Versailles by the United States Senate was a very drawn out and complicated process, the failure of ratification can be reduced to two simple concepts. First, the discussion of the treaty can be reduced to the traditional haggling of bipartisan politics of the United States. Woodrow Wilson, a democrat, had a vision for peace that was called the Fourteen Points. He was the man responsible for negotiating the Treaty of Versailles on behalf of the United States. The trouble is, while he was the one sent to France, Constitutionally, he does not have the authority to ratify treaties. The right is given to the Senate. The Senate was led by the other party, the Republicans. In fact, Henry Cabot Lodge was a very powerful voice in the Senate and he was Woodrow Wilson's primary political enemy. It would be hard to imagine that Lodge and Wilson would agree on much, let alone a major peace agreement that would affect the future of the United States on the world stage.
A second reason that the Treaty did not get passed was because of the creation of the League of Nations. The League of Nations was the predecessor to the United Nations as a world organization for peace. It was established by the Treaty of Versailles and was one of Wilson's Fourteen Points. Lodge and other members of the Senate felt that the United States would lose its authority to declare war unilaterally if it entered the League of Nations. They also were responding to the popular sentiment of isolationism amongst the American people after the war. Inclusion in the League of Nations had the potential of entangling the United States in the affairs of Europe. The creation of the League of Nations was the primary sticking point for the treaty being ratified. As a result, the United States would sign a separate peace agreement with Germany in 1921.