Although the League of Nations was Woodrow Wilson's own idea, and the one part of his fourteen points which he cared so deeply about, when the treaty was presented to Congress, it wasn't ratified. There was much opposition to the League of Nations in the Senate, notably from Senator Henry Cabot Lodge. Lodge argued:

"But it is well to remember that we are dealing with nations every one of which has a direct individual interest to serve, and there is grave danger in an unshared idealism." (full speech can be found here http://www.firstworldwar.com/source/lodge_leagueofnations.htm)

Another reason the treaty of versailles failed to be passed in Senate was because President Woodrow Wilson would not allow it to be reformed in the slightest. He took the approach that it had to be ratified exactly as it was, with no changes what so ever. For this reason, the people who wished to reform the treaty ended up voting against it.