As we all know, the United States very nearly didn't sign the Treaty of Versailles. For those of you who need a history lesson, Willson quite annoyed Congress, and there was a lot of talk of Congress not ratifying the Treaty, and forming a separate peace with the Central Powers. If this had happened, it quite likely that they wouldn't have joined the League of Nations. What would the implications of this be?
EDIT: To be better (in response to Blue Max's very correct points)
As we all know, the United States very nearly didn't sign the Treaty of Versailles. For those of you who need a history lesson, a combination of President Wilson's clearly sectarian politics, the heavy losses the US took during the war (yes, light compared to Europe's, but...), and the dedicated campaigning of a number of prominent senators nearly prevented this event. This same bloc also spoke against joining the League of Nations, and had they won the first battle, they almost certainly would have taken the second as well. Had this occurred, what sort of effects might have come about?