The League of Nations was not a success because it could not prevent war and never gained United States participation. In 1908, U.S. President Woodrow Wilson proposed the idea for the League of Nations when he called for a:
general association of nations…formed under specific covenants for the purpose of affording mutual guarantees of political independence and territorial integrity to great and small states alike.
Wilson submitted the charter for the League to Congress, who--led by Henry Cabot Lodge--voted it down because they feared becoming intertwined with Europe. This was embarrassing for Wilson, but the lack of U.S. participation also, according to the U.S. State Department, caused the League to operate "much less effectively without U.S. participation than it would have otherwise."
Most notably, the League was not able to fulfill its primary goal, which was to prevent another conflict akin to the Great War, for World War II began just two decades after the League's creation in 1920. Recognizing its deficiencies, the world scrapped the League following World War II (1946), replacing it with the United Nations.