The U.S. won World War I and set up the League of Nations to decide on what to do with Germany and Italy. The Treaty of Versailles was also signed that set limits on the future of Germany. America did not sign due to unrest at home. Actually- The League of Nations was a condition contained within the treaty of Versailles. Because of a lack of support in Congress, the United States never signed this document and therefore never entered this collaboration between nations. Due to the United States' lack of participation, the League of Nations was not long lived, and not until the creation of the United Nation (WWII) did the United States become involved in an international coalition such as this.
