Containment was the foreign policy of the United States from the late 1940s through most of the 1950s. It was an attempt to halt what US policymakes viewed as Soviet expansionism, especially in Europe, in the wake of World War II. The term was first used in 1947 by George Kennan in a famous anonymous article (known as the "X Article" but actually entitled "The Sources of Soviet Conduct") in the journal Foreign Affairs. It was a continuation of a number of ideas he had first advanced in a "Long Telegram" to the U.S. State Department from his diplomatic post in Moscow in 1946. This telegram argued that the Soviets had designs on all of Eastern Europe, but would yield in the face of firm resistance.
In the "X Article" he thus advocated a "patient but firm and vigilant containment of Russian expansive tendencies." The United States must, he argued, counter "Soviet pressure against the free institutions of the Western world" through the "adroit and vigilant application of counter-force at a series of constantly shifting geographical and political points, corresponding to the shifts and maneuvers of Soviet policy." Kennan thought this policy would generate failures for the Soviet Union that might weaken the entire edifice upon which Soviet power was based, the inevitabilty of world communist domination.
The policy was put into practice by President Harry Truman, who, in pursuance of the so-called "Truman Doctrine," sent millions in aid to both Greece and Turkey, which were in the midst of civil strife and facing communist takeover. US intervention in Korea was also an example of containment. While the policy came under criticism in the early fifties, it would remain influential in the thinking of many policy-makers throughout the Cold War.