D.United States
The US refused to join the League of Nations because of a philosophy that had long existed in America that is known as "Isolationism". The concept of Isolationism gradually evolved from the beginnings of the United States of America.
The United States is the nation that refused to join the League of Nations due to a prevailing isolationist philosophy and concerns about losing congressional authority over military decisions. The U.S. Senate opposed the Treaty of Versailles, which included the League, leading to the nation's absence in this international organization. This decision ultimately weakened the League's effectiveness in maintaining peace and resolving conflicts.
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