Wilson wanted the United States to join the League of Nations, but the US Senate didn't pass the Treaty of Versailles, so the US didn't join the League of Nations.

You may be interested in J. Michael Hogan's *Woodrow Wilson's Western Tour,* which details and analyzes Wilson's "stumping" across the country to try to drum up popular support for the League. It was a turning point in what we consider the "rhetorical presidency": knowing that Congress did not support his initiative, Wilson took his case to the people in hopes that they would pressure their representatives to ratify the treaty. Nowadays, we expect this sort of thing from our elected leaders, but at the time it was still a somewhat radical notion that the President would try to engage the support of the people for his policies, rather than working with and through Congress (T. Roosevelt did this as well, but before him it was relatively unheard of). Ultimately, as Hogan argues, the tour not only failed to accomplish Wilson's goals, but destroyed his health as well -- plus Congress was unlikely to ratify the treaty even in the face of popular support for it.

Article 10 was the main reason. Article 10 basically said that any member of the league would go to war should the league deem it necessary. Congress was against this because it limits their power to declare war, which, in turn gives the president more power and offsets checks and balances by a fair bit.