How the Marshall plan helped enforce the Truman Doctrine

As suggested by the names, the Truman Doctrine represented a manifesto of ideas for international relations after World War II, while the Marshall Plan was a strategy for enforcing these ideas. The primary point of the Truman Doctrine was an example of American exceptionalism: it suggested that it was up to the United States to contain the spread of Communism worldwide, even in places that were unrelated to the US. This was because the US felt threatened by the spread of Communism across Eastern Europe after the war.
General Marshall was dispatched by Truman to investigate what the situation was in Europe, and to report on why he felt the countries were at such threat of becoming Communist. His assessment was that the destruction caused by the war had left them so impoverished that Communism was extremely appealing to them as the only possible option. As a result, Marshall was able to convince Congress to donate significant amounts of money to the regeneration of Europe in the form of money and food aid. The US thus contributed to the rebuilding of Europe and the countries who had been helped owed a debt to the US and saw it—rather than the Communist USSR—as the benevolent power.
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