After World War II, the United States had a democratic system with multiple political parties, while the Soviet Union operated under a single-party system led by the Communist Party. This fundamental difference contributed to the tensions of the Cold War. Therefore, the correct choice is option D.
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After World War II, the United States had a democratic system characterized by multiple political parties, while the Soviet Union operated under a single-party system led by the Communist Party. This difference was central to the Cold War conflict that followed. Thus, the correct description is that the United States had a democratic system, and the Soviet Union had a single-party system. ;