President Truman was very concerned by the growth of Soviet power. He realised that the USA could no longer continue with its policy of isolationism.
If the spread of communism was to be halted, Truman believed that the USA would have to be much more active in world affairs.
This policy became known as the Truman Doctrine.
American and British politicians were concerned about events in Greece and Turkey.
From 1946 a civil war had been happening in Greece between its western-backed government and pro-communist forces.
Truman had to persuade the usually isolationist US government to grant American money to support Greece’s fight against communism and laid out his reasons in a speech to Congress in March 1947.
In that speech he promised that the USA would provide aid to any country standing up to oppression: He stated, "it must be the policy of the United States to support free peoples who are resisting attempted subjugation [takeover] by armed minorities or by outside pressures."
It was clear that Truman was referring to countries resisting an armed communist takeover, but it was not spelt out in such straightforward terms and so gave the US government opportunity to take action where they felt they needed to step in.
America was now committed to a policy of containment, or stopping the spread of communism.