The Truman Doctrine and The Marshall Plan were both party of the US foreign policy strategy of containment, or as I like to call it The Anti-Communism Strategy. The idea of containment was to contain and prevent the spread of communism world-wide during the Cold War. In fact, the Truman Doctrine, which states that the US will provide political, military and economic assistance to any democratic nation threatened by communism, is believed to be part of the reason the US got involved in Vietnam, despite the problem not particularly being communism.
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