The League of Nations was an idea similar to the United Nations but the United States didn't want to participate in them. United Nations came into existence after world war 2 and D is NATO which is a military alliance. The Warsaw Pact was completely different and related to the Eastern Bloc.
Explanation: The League of Nations was created by Woodrow Wilson, so the Senate’s rejection of it was a severe blow to the prestige of the President and the reputation of the United States.