This said that the United States would aid any nation in resisting the growing
threat of communism and became the guiding force of American foreign
policy during the Cold War.

Truman Doctrine

Explanation:

The Cold War was a decades-long conflict, essentially between democracy and communism (or the US and the Soviet Union, if we're talking about specific countries). Basically, the US wanted to prevent more nations in the world from converting their governments into a communist one as such a move would likely lead to grave economic and social issues in the future.

At the beginning, Harry S. Truman was the President of the US. During this time, he proposed the Truman Doctrine, which stated that it would try to contain the Soviet's expansion of communism ideals and aid nations who wanted to resist these ideals.

Thus, the answer is the Truman Doctrine.

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