Truman doctrine

The US was concerned that European powers might try to recolonise Latin America, and the Monroe Doctrine stated that any such attempt would be regarded as aggression against the United States itself.

[I]n one sense, the Monroe Doctrine, although originally only applicable to the Western hemisphere, is a precedent for the Truman doctrine. The Monroe Doctrine […] was designed to exclude European colonization and European systems from any portion of the western hemisphere [and] promised that America would [do likewise to Europe]. It has been suggested that President Truman has merely projected the Monroe Doctrine into […] the atomic age. […] President Truman's proposal could also be interpreted as running counter to the [idea] that America would not interfere with Europe.

These policies [which Truman says have "averted World War III" and "attain[ed] peace" in his farewell address]—notably, the Truman Doctrine, the Marshall Plan, and the North Atlantic Treaty Organization—and the geopolitical conditions they created made up […] the strategy of containment.