Probably the best account of the contortions surrounding the League of Nations comes in Margaret Macmillan's Paris 1919 (alternate title: The Peacemakers.) To give a short version:

The League of Nations was the brainchild of US President Woodrow Wilson as part of the peace settlement for the First World War. This was initially, ostensibly, an extremely popular move; Wilson was greeted in Paris for the 1919 Paris Peace Conference as an enlightened liberator by much of French society, and there was genuine popular feeling that he might be a true statesman and peacemaker to bring order to the tremendous postwar chaos.

Three intertwined problems, however, stood in the way of Wilson in doing so. The first was that he was himself wildly contradictory. For instance, one of his famous Fourteen Points was the idea of "national self-determination". This basically allowed for people to decide for themselves what nation they wished to belong to -- thus, Czech patriots could declare Czech independence from Austria-Hungary, and so on. The problem with this was that Wilson himself was unsure what he meant by "national self-determination" (he quite often changed his position on it in discussions with his closest confidant, Colonel House.) He was also unwilling to apply it to the United States' own overseas territories, in which there were undoubtedly nascent nationalist movements. The spirit of the idea also caused significant problems in areas of mixed ethnicity, and it was blatantly ignored in some cases. For instance, the final Treaty of Versailles insisted that Germany and Austria could not join together in spite of the fact that most Austrians defined themselves as ethnically German; though there were obvious reasons to not allow the two major Central Powers to unite after their defeat in the First World War, this demonstrated an inherent contradiction in the idea.

The second problem Wilson faced came from his own allies, the British and the French. It was all very well for an American president to go to Europe and claim that the Americans would set everything right, but this ignored the fact that Europe (particularly France in this case) had been subjected to years of debilitating and destructive war. French industry and agriculture, for example, had been smashed. Wilson advocated peace without victory, and peace with honour, but in a realistic sense this could not be achieved, because (as Georges Clemenceau rightly pointed out) the Allies had incurred massive debts, and had taken such significant losses, that it was reasonable for them to expect to be able to squeeze Germany.

All this made the Paris Peace Conference very difficult for Wilson, and the perception of these intractable difficulties damaged his own political brand. This leads to our third problem: Wilson was a ham-fisted politician with a messianic complex. The previous points show his naïveté. But Wilson was also personally priggish, and very difficult to work with. It was controversial that he had even gone to Paris himself, since this was the first time a sitting US president had ever left the country; some members of Congress even wondered if he could be impeached for doing so. But it was also felt that, whatever decisions were made, they should be non-partisan, since they impacted all of America, not just Wilsonian Democrats. But Wilson operated a sort of personal diplomacy, relying an awful lot on his own individual characteristics, and really taking advice only from House as well as his new wife, Edith Bolling Galt Wilson. To this end, he often ignored the counsel of his specialists, such as the army chief John Pershing, or his secretary of state Robert Lansing; Lansing and Wilson did not get along at all. Somewhat worse, Wilson did not recognise his need to keep good favour with Republicans at home; when he took a break from the conference and returned home, he chose to give a public speech extolling his plans for the peace in Boston. He did not consult Congress. Even worse, Boston was the home of his great political rival, Henry Cabot Lodge, and the speech, at which drafts of the Covenant for the League of Nations were distributed to the audience, was viewed as a serious breach of etiquette.

All this amounted to a serious problem for Wilson, and particularly his "baby" -- the proposed League of Nations. In Paris he drove negotiations for its creation. At home, however, the continued problems of the conference renewed sentiment that America had been drawn into the affairs of other states, and had suffered because of it. Naturally resulting from this was a sense of isolationism. Adding to this, Wilson's growing unpopularity among the electorate and the politicians (both Democrat and Republican) led to a type of anti-Wilsonian opposition among the powerful movers and shakers in Washington, such as Lodge, or William Jennings Bryan. As a result, as the League was being formed, its key backer (Wilson) no longer had the confidence of the country he led. Because of this, when the League came into existence, America's membership was not ratified, and the United States never joined. Instead of becoming a leading member of the prototypical international community, the US largely withdrew from world affairs and -- politically, at least -- remained isolationist for another two decades.

I'm not quite sure what your question is asking, the United States never joined the League of Nations in the first place.