On November 19, 1919, the United States Senate rejected the Treaty of Versaille which formally ended World War I. It is important to remember that, although the president negotiates foreign treaties, it is up to the Senate to ratify them with a two-thirds approval. This ultimately proved to be too high a threshold.
Henry Cabot Lodge, the Massachusetts senator and Republican majority leader (President Wilson was a Democrat) had serious reservations about the provision in the Treaty of Versailles that called for the creation of the League of Nations. He felt that being part of this international body would mean giving up too much autonomy. Lodge responded with his "14 Reservations" to counter Wilson's "14 Points." These reservations would place limits on the country's role in the League of Nations.
Even more staunchly against the Treaty of Versailles were a group of senators who became known as the "irreconcilables." These senators, led by William Edgar Borah of Idaho opposed the Treaty of Versailles completely, regardless of Senator Lodge's reservations.
When it came to vote on November 19, Wilson urged his supporters to vote against the version with the reservations. With the irreconcilables also voting against it, the resolution only garnered 55 votes in favor, short of the two-thirds required. When a version of the resolution without the reservations was brought up, it too failed to reach a two-thirds majority, getting just 53 favorable votes, this time with Wilson's supporters voting in favor and Lodge's voting against it.
As a result of these political divisions, the United States never ratified the Treaty of Versailles despite being instrumental in drafting it.