The correct answer to this open question is the following.
The United States 'Truman Doctrine' was the foreign policy of the United States established by US President Harry Truman in 1947. After its participation in World War I, President Truman decided to be more actively involved in world affairs, in the opposite direction of his predecessors that preferred to maintain the foreign policy of neutrality or isolationism.
President Truman said that the United States would offer economic, political, and military assistant to all democratic nations in need.
This new policy is similar to the U.S. 'Domino Theory' and the U.S. idea of 'Containment' in that this new international approach was to make a solid presence in the world in the time of the so-called Cold War. A time in modern history when the Soviet Union tried to spread Communism all over the world, while the United States tried to stop it by establishing the policy of containment.
During the Cold War, the USSR and the US competed in the arms race, the space race, and the spread/containment in places such as North Korea/South Korea, North Vietnam/South Vietnam, Eastern Eure, and Cubal.