President Woodrow Wilson came up with points during World War I called the Fourteen Points. These were his goals for during and after the war, included in these points was the League of Nations. When other countries did approve of the League of Nations, along with President Woodrow Wilson himself, the United States Congress did not want the United States to join.
Congress did not want the United States to join the League of Nations because they believed in isolationism. This means that they did not want to get involved in foreign affairs whatsoever and wanted to mind their business. They believed that the "domino effect" would happen, as it did with the start of World War I by numerous countries getting involved over another countries problems.
The United Nations was much more powerful than the League of Nations. The League of Nations proved to be very ineffective, they did not have their own troops, and they couldn't stop Germany from expanding. At the end of World War II, which the League of Nations failed to prevent, the United Nations was created. The United Nations proved to be powerful and much more effective than the League of Nations was. Numerous countries joined the United Nations, including the United States, as there were fewer isolationists and the United Nations actually held power.