The reason why the U.S. followed an isolationist foreign policy during the late 1910s and early 1920s was primarily due to:
A. America's involvement in World War I had been costly and deadly.
During World War I, the United States suffered significant casualties and incurred high financial costs. This experience made many Americans hesitant to engage in international conflicts or alliances that could lead to further loss of life and resources.
Additionally, the U.S. had traditionally followed a policy of non-intervention in European affairs, as seen in option D. The nation aimed to maintain its neutrality and avoid entangling alliances with European powers, reflecting a desire to prioritize its own interests and avoid being drawn into conflicts abroad.