What was the main action that sparked the us declaration of the truman doctrine?

It was communism that sparked the Truman doctrine. In 1947 the British Government said they could no longer afford to help Greece and Turkey fight communism both economically or militarily. President Truman believing these countries were in a danger of communism then created the Truman doctrine, which was a promise that the USA would do whatever necessary both economically and militarily to prevent communism from spreading.

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the name of the theory is landmass theory.