Containment
Earlier uses of term: Both Americans and Europeans were aware with significant historical antecedents. In the 1850s, anti-slavery forces in the United States developed a free soil strategy of containment to stop the expansion of slavery until it later collapsed. Historian James Oakes explains the strategy:

The Federal government would surround the south with free states, free territories, and free waters, building what they called a 'cordon of freedom' around slavery, hemming it in until the system's own internal weaknesses forced the slave states one by one to abandon slavery.
Between 1873 and 1877, Germany repeatedly intervened in the internal affairs of France's neighbors. In Belgium, Spain, and Italy, Chancellor Otto von Bismarck exerted strong and sustained political pressure to support the election or appointment of liberal, anticlerical governments. That was part of an integrated strategy to promote republicanism in France by strategically and ideologically isolating the clerical-monarchist regime of President Patrice de MacMahon. It was hoped that by surrounding France with a number of liberal states, French Republicans could defeat MacMahon and his reactionary supporters. The modern concept of containment provides a useful model for understanding the dynamics of this policy.
After the 1917 October Revolution in Russia, there were calls by Western leaders to isolate the Bolshevik government, which seemed intent on promoting worldwide revolution. In March 1919, French Premier Georges Clemenceau called for a cordon sanitaire, a ring of non-communist states, to isolate Soviet Russia. Translating that phrase, US President Woodrow Wilson called for a "quarantine."
The World War I allies launched an incursion into Russia, as after the Bolshevik Revolution, Vladimir Lenin withdrew the country from the First World War, allowing Germany to reallocate troops to face the Allied forces on the Western Front. Concurrently, President Wilson became increasingly aware of the human rights violations perpetuated by the new Russian Soviet Federative Socialist Republic, and opposed the new regime's militant atheism and advocacy of a command economy. He also was concerned that Marxism–Leninism would spread to the remainder of the Western world, and intended his landmark Fourteen Points partially to provide liberal democracy as an alternative worldwide ideology to Communism. Despite reservations, the United States, as a result of the fear of Japanese expansion into Russian-held territory and their support for the Allied-aligned Czech Legion, sent a small number of troops to Northern Russia and Siberia. The United States also provided indirect aid such as food and supplies to the White Army. The incursion was unpopular at home and lacked a cohesive strategy, leading the allies to ultimately withdraw from Russia.
The U.S. initially refused to recognize the Soviet Union, but President Franklin D. Roosevelt reversed the policy in 1933 in the hope to expand American export markets.
The Munich Agreement of 1938 was a failed attempt to contain Nazi expansion in Europe. The U.S. tried to contain Japanese expansion in Asia from 1937 to 1941, and Japan reacted with its attack on Pearl Harbor.
After Germany invaded the Soviet Union in 1941 during World War II, the U.S. and the Soviet Union found themselves allied against Germany and used rollback to defeat the Axis powers: Germany, Italy, and Japan.

Origin (1944–1947): Key State Department personnel grew increasingly frustrated with and suspicious of the Soviets as the war drew to a close. Averell Harriman, U.S. Ambassador in Moscow, once a "confirmed optimist" regarding U.S.–Soviet relations, was disillusioned by what he saw as the Soviet betrayal of the 1944 Warsaw Uprising as well as by violations of the February 1945 Yalta Agreement concerning Poland. Harriman would later have a significant influence in forming Truman's views on the Soviet Union.
In February 1946, the U.S. State Department asked George F. Kennan, then at the U.S. Embassy in Moscow, why the Russians opposed the creation of the World Bank and the International Monetary Fund. He responded with a wide-ranging analysis of Russian policy now called the Long Telegram:

 Soviet power, unlike that of Hitlerite Germany, is neither schematic nor adventuristic. It does not work with fixed plans. It does not take unnecessary risks. Impervious to the logic of reason, and it is highly sensitive to the logic of force. For this reason, it can easily withdraw—and usually does when strong resistance is encountered at any point.
Kennan's cable was hailed in the State Department as "the appreciation of the situation that had long been needed." Kennan himself attributed the enthusiastic reception to timing: "Six months earlier the message would probably have been received in the State Department with raised eyebrows and lips pursed in disapproval. Six months later, it would probably have sounded redundant." Clark Clifford and George Elsey produced a report elaborating on the Long Telegram and proposing concrete policy recommendations based on its analysis. This report, which recommended "restraining and confining" Soviet influence, was presented to Truman on September 24, 1946.
In January 1947, Kennan drafted an essay entitled "The Sources of Soviet Conduct." Navy Secretary James Forrestal gave permission for the report to be published in the journal Foreign Affairs under the pseudonym "X." Biographer Douglas Brinkley has dubbed Forrestal "godfather of containment" on account of his work in distributing Kennan's writing. The use of the word "containment" originates from this so-called "X Article": "In these circumstances, it is clear 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 later turned against the containment policy and noted several deficiencies in his X Article. He later said that by containment he meant not the containment of Soviet Power "by military means of a military threat, but the political containment of a political threat." Second, Kennan admitted a failure in the article to specify the geographical scope of "containment", and that containment was not something he believed the United States could necessarily achieve everywhere successfully.

Harry S. Truman: After Republicans gained control of Congress in the 1946 elections, President Truman, a Democrat, made a dramatic speech that is often considered to mark the beginning of the Cold War. In March 1947, he requested that Congress appropriate $400 million in aid to the Greek and Turkish governments, which were fighting communist subversion. Truman pledged to, "support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures." This pledge became known as the Truman Doctrine. Portraying the issue as a mighty clash between "totalitarian regimes" and "free peoples", the speech marks the adoption of containment as official US policy. Congress appropriated the money.
Truman's motives on that occasion have been the subject of considerable scholarship and several schools of interpretation. In the orthodox explanation of Herbert Feis, a series of aggressive Soviet actions in 1945–1947 in Poland, Iran, Turkey, and elsewhere awakened the American public to the new danger to freedom to which Truman responded. In the revisionist view of William Appleman Williams, Truman's speech was an expression of longstanding American expansionism. In the realpolitik view of Lynn E. Davis, Truman was a naive idealist who unnecessarily provoked the Soviets by couching disputes in terms like democracy and freedom that were alien to the communist vision.
According to a psychological analysis by Deborah Larson, Truman felt a need to prove his decisiveness and feared that aides would make unfavorable comparisons between him and his predecessor, Roosevelt. "I am here to make decisions, and whether they prove right or wrong I am going to take them", he once said.
The drama surrounding the announcement of the Truman Doctrine catered to the president's self-image of a strong and decisive leader, but his real decision-making process was more complex and gradual. The timing of the speech was not a response to any particular Soviet action but to the fact that the Republican Party had just gained control of Congress. Truman was little involved in drafting the speech and did not himself adopt the hard-line attitude that it suggested until several months later.
The British, with their own position weakened by economic distress, urgently called on the U.S. to take over the traditional British role in Greece. Undersecretary of State Dean Acheson took the lead in Washington, warning congressional leaders in late February 1947 that if the United States did not take over from the British, the result most probably would be a "Soviet breakthrough" that "might open three continents to Soviet penetration." Truman was explicit about the challenge of communism taking control of Greece. He won wide support from both parties as well as experts in foreign policy inside and outside the government. It was strongly opposed by the left, notably by former Vice President Henry A. Wallace, who ran against Truman in the 1948 presidential campaign.
Truman, under the guidance of Acheson, followed up his speech with a series of measures to contain Soviet influence in Europe, including the Marshall Plan, or European Recovery Program, and NATO, a 1949 military alliance between the U.S. and Western European nations. Because containment required detailed information about communist moves, the government relied increasingly on the Central Intelligence Agency (CIA). Established by the National Security Act of 1947, the CIA conducted espionage in foreign lands, some of it visible, more of it secret. Truman approved a classified statement of containment policy called NSC 20/4 in November 1948, the first comprehensive statement of security policy ever created by the United States.