The Truman Doctrine was policy developed by President Harry S. Truman in 1947. It established the precedent that the United States will intervene in crises that involve democratic countries. These crises, either internally or externally, could include political, military, or economic assistance to the beleaguered countries.

The idea was delivered in a Congressional address, stimulated by recent developments in the conflict between the Greek government and the Greek Communist Party, in which the British government decided to remove their aid to the country. Truman asked the joint-Congressional committee that had assembled to give more aid to the Greek government.

In this act, Truman reversed policy on foreign affairs. Previous to this policy development, America's association with other countries was dictated by the Monroe Doctrine, which essentially stated that the United States would not intervene in foreign affairs that did not concern the country directly. It led to a larger global presence of the United States in foreign affairs, and helped ensure a more global understanding of other countries. It also helped develop a comprehensive array of allies for the United States, and was demonstrative in such concepts as trade, cultural exchange, and intelligence interchange.