The final official stance of the United States was that it would not join the League of Nations.  It is true that President Wilson first proposed the League, but presidents cannot commit the US to membership in such organizations on their own.  When a treaty is needed to join an organization, the Senate must confirm that treaty.  The Senate in this case did not confirm the treaty and the US never joined the League of Nations.

The main reason for this was a fear that the US would lose its sovereignty.  Many in the Senate feared that membership in the League of Nations would force the United States to enter wars that were not in its national interest.  They wanted to be sure that the US would be able to opt out of any wars that the League of Nations declared.  President Wilson did not want to allow any modifications to the treaty.  For this reason, he refused to accommodate the wishes of these senators and the treaty was never ratified.  This is why the US did not end up participating in the League of Nations even though it was Wilson’s brainchild.