American foreign policy under the Truman Administration was largely guided by the principle of containment. This doctrine was first proposed by foreign policy analyst George Kennan, who proposed that
the main element of any United States policy toward the Soviet Union must be that of long-term, patient but firm and vigilant containment of Russian expansive tendencies.
Kennan argued that the Soviet Union, driven by a combination of historical forces and communist ideology, was inherently expansionist, but that it could be checked, perhaps without a large-scale confrontation, by firmly resisting every one of its efforts to expand its influence. His analysis seemed particularly accurate as the Grand Alliance began to fracture after World War II.
By 1947, the Soviet Union had established communist governments in almost all of Eastern Europe, and with Greece and Turkey in the midst of potential communist takeovers, the Truman Administration gave millions of dollars in aid to anti-communist politicians and fighters in those countries. The aid had its desired effect, and the policy of assisting anti-communist forces as part of containment became known as the Truman Doctrine. By 1948, the United States had implemented the Marshall Plan, which allocated billions in aid to European nations in an effort to check the spread of communism through economic rehabilitation.
Containment had a military face as well, as the United States led a United Nations intervention on the Korean peninsula to stop the invasion of Communist forces from North Korea. But whether military, economic, or diplomatic, the guiding principle of US foreign policy during this period was based on containment.