the cold war
The Cold War was the period of conflict, tension and competition between the United States and the Soviet Union and their respective allies from the mid-1940s until the early 1990s. Throughout this period, the rivalry between the two superpowers unfolded in multiple arenas: military coalitions; ideology, psychology, and espionage; sports; military, industrial, and technological developments, including the space race; costly defence spending; a massive conventional and nuclear arms race; and many proxy wars.
There was never a direct military engagement between the US and the Soviet Union, but there was half a century of military buildup as well as political battles for support around the world, including significant involvement of allied and satellite nations in proxy wars. Although the US and the Soviet Union had been allied against Nazi Germany, the two sides differed on how to reconstruct the postwar world even before the end of World War II. Over the following decades, the Cold War spread outside Europe to every region of the world, as the US sought the "containment" of communism and forged numerous alliances to this end, particularly in Western Europe, the Middle East, and Southeast Asia. There were repeated crises that threatened to escalate into world wars but never did, notably the Berlin Blockade (1948-1949), the Korean War (1950-1953), the Vietnam War (1959-1975), the Cuban Missile Crisis (1962), and the Soviet-Afghan War (1979-1989). There were also periods when tension was reduced as both sides sought détente. Direct military attacks on adversaries were deterred by the potential for mutual assured destruction using deliverable nuclear weapons.
The Cold War drew to a close in the late 1980s following Soviet leader Mikhail Gorbachev's summit conferences with United States President Ronald Reagan, as well as Gorbachev's launching of reform programs: perestroika and glasnost
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In the specific sense of the Cold War referring to the post-World War II geopolitical tensions between the Soviet Union and the United States, the term has been attributed to American financier and U.S. presidential advisor Bernard Baruch. The Cassell Companion to Quotations cites a speech Baruch gave in South Carolina, April 16, 1947 in which he said, "Let us not be deceived: we are today in the midst of a cold war." The Cassell Companion notes that the phrase was actually suggested to Baruch by his speechwriter, Herbert Bayard Swope, who had been using it privately since 1940. Columnist Walter Lippmann also gave the term wide currency after his 1947 book titled Cold War.
The ideological clash between communism and capitalism began in 1917 following the Russian Revolution, when the USSR emerged as the first major communist power. This was the first event which made Russian-American relations a matter of major, long-term concern to the leaders in each country.
Several events led to suspicion and distrust between the United States and the Soviet Union: US intervention in Russia supporting the White Army in the Russian Civil War, Russia's withdrawal from World War I in the Treaty of Brest-Litovsk with Germany, the Bolsheviks' challenge to capitalism, the US refusal to recognize the Soviet Union until 1933. Other events in the period immediately before WWII increased this suspicion and distrust. The British appeasement of Germany and the German-Soviet Non-aggression Pact are two notable examples.
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During the war, the Soviets strongly suspected that the Anglo-Americans had opted to let the Russians bear the brunt of the war effort, to insert themselves only at the last minute so as to influence the peace settlement and dominate Europe. Historians such as John Lewis Gaddis dispute this claim, citing other military and strategic calculations for the timing of the Normandy invasion. Nevertheless, Soviet perceptions (or misconceptions) of the West and vice versa left a strong undercurrent of tension and hostility between the Allied powers.
There was severe disagreement between the Allies about how Europe should look following the war. Both sides, moreover, held very dissimilar ideas regarding the establishment and maintenance of post-war security. The Americans tended to understand security in situational terms, assuming that, if US-style governments and markets were established as widely as possible, countries could resolve their differences peacefully, through international organizations. Soviet leaders, however, tended to understand security in terms of space. This reasoning was conditioned by Russia's historical experiences, given the frequency with which the country had been invaded over the last 150 years.
At the Yalta Conference in February 1945, the Allies attempted to define the framework for a post-war settlement in Europe but could not reach a firm consensus. Following the Allied victory in May, the Soviets effectively occupied Eastern Europe, while the US had much of Western Europe. In occupied Germany, the US and the Soviet Union established zones of occupation and a loose framework for four-power control with the ailing French and British.
At the Potsdam Conference, starting in late July, serious differences emerged over the future development of Germany and Eastern Europe. At this conference Truman informed Stalin that the United States possessed a powerful new weapon. "Stalin’s only reply was to say that he was glad to hear of the bomb and he hoped [the United States] would use it."  One week after the end of the Potsdam Conference, the atomic bombings of Hiroshima and Nagasaki led to further conflict between the Soviet Union and the United States. Shortly after the attacks, Stalin protested to US officials when Truman offered the Soviets little real influence in occupied Japan.
In February 1946, George F. Kennan's "Long Telegram" from Moscow helped to articulate the growing hard line that was being taken against the Soviets.. On September 6, 1946, James F. Byrnes made a speech in Germany, repudiating the Morgenthau Plan and warning the Soviets that the US intended to maintain a military presence in Europe indefinitely. (see Restatement of Policy on Germany) As Byrnes admitted one month later, "The nub of our program was to win the German people [...] it was a battle between us and Russia over minds [....]" A few weeks after the release of this "Long Telegram", former British Prime Minister Winston Churchill delivered his famous "Iron Curtain" speech in Fulton, Missouri. The speech called for an Anglo-American alliance against the Soviets, whom he accused of establishing an "iron curtain" from "Stettin in the Baltic to Trieste in the Adriatic."
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By 1947, Truman's advisors were worried that time was running out to counter the influence of the Soviet Union. In Europe, post-war economic recovery was faltering, and shortages of food and other essential consumer goods were common. Truman's advisors feared that the Soviet Union was seeking to weaken the position of the US in a period of post-war confusion and collapse.
The event which spurred Truman on to announce formally the US's adopting the policy of "containment" was the British government's announcement in February 1947 that it could no longer afford to finance the Greek monarchical military regime in its civil war against communist-led insurgents. (See Greek Civil War.) Rather than view this war as a civil conflict revolving around domestic issues, US policymakers interpreted it as a Soviet effort; however, the insurgents were helped by Josip Broz Tito's Yugoslavia, not Moscow. Secretary of State Dean Acheson accused the Soviet Union of conspiracy against the Greek royalists in an effort to "expand" into the Middle East, Asia, and Africa, and in March 1947 the administration unveiled the "Truman Doctrine". It "must be the policy of the United States," Truman declared, "to support free peoples who are resisting attempted subjugation by armed minorities or outside pressures."
Truman rallied Americans in his famous "Truman Doctrine" speech to spend $400,000,000 on intervention in the civil war in Greece. In order to mobilize an unfriendly Republican Congress, the Democratic president painted the conflict as a contest between "free" peoples and "totalitarian" regimes, thus dramatically heightening the rhetorical stakes of the conflict. By aiding Greece, Truman set a precedent for US aid to regimes, no matter how repressive and corrupt, that requested help to fight communists. 
Without the assistance of huge capital resources to rebuild industry transferred from the United States, Western European economies failed to recover from the enormous wartime destruction of the region's infrastructure. Communist parties, meanwhile, were winning large votes in free elections in countries such as France and Italy. American policymakers were worried that economic conditions in Western Europe might deteriorate to the point where communist parties could seize power there, too, through free elections or popular revolutions. Some US policymakers also feared that their own economy might suffer unless effective demand for their exports in Western Europe was restored.
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For US policymakers, threats to Europe's balance of power were not necessarily military ones, but a political and economic challenge. George Kennan helped to summarise the problem at the State Department Planning Staff in May 1947: "Communist activities" were not "the root of the difficulties of Western Europe" but rather "the disruptive effects of the war on the economic, political, and social structure of Europe." According to this view, the Communists were "exploiting the European crisis" to gain power. In June, following the recommendations of the State Department Planning Staff, the Truman Doctrine was complemented by the Marshall Plan, a pledge of economic assistance aimed at rebuilding the Western political-economic system and countering perceived threats to Europe's balance of power, which the US had gone to war to restore, from the radical left.
After lobbying by the Joint Chiefs of Staff, and Generals Clay and Marshall, the Truman administration finally realised that economic recovery in Europe could not go forward without the reconstruction of the German industrial base on which it had previously been dependent.
In July, Truman rescinded, on "national security grounds", the punitive Morgenthau plan JCS 1067, which had directed the US forces of occupation in Germany to "take no steps looking toward the economic rehabilitation of Germany." It was replaced by JCS 1779, which stressed instead that "[a]n orderly, prosperous Europe requires the economic contributions of a stable and productive Germany."
Also in July, Truman reorganised his government to fight the Cold War. The National Security Act of 1947, signed by Truman on July 26, created a unified Department of Defense, the Central Intelligence Agency (CIA), and the National Security Council. These would become the main bureaucracies for US policy in the Cold War.
The twin policies of the Truman Doctrine and the Marshall Plan led to billions in economic and military aid to Western Europe, and Greece and Turkey. With US assistance, the Greek military won its civil war, and the Italian Christian Democrats defeated the powerful Communist-Socialist alliance in the elections of 1948.
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The US consolidated its new role as leader of the West. In retaliation to Western moves to reunite West Germany, Stalin built blockades to block western access to West Berlin, but Truman maintained supply lines to the enclave by flying supplies in over the blockade from 1948 to '49. (see Berlin Blockade)
The US formally allied itself to the Western European states in the North Atlantic Treaty of 1949, establishing the North Atlantic Treaty Organization (NATO). Stalin countered by tying together the economies of the Eastern bloc in a Soviet-led version of the Marshall Plan, the Council for Mutual Economic Assistance (COMECON), and exploding the first Soviet atomic device in August 1949.
The US took the lead in re-establishing West Germany from the three Western zones of occupation in 1949. To counter this Western reorganisation of Germany, the Soviet Union proclaimed its zone of occupation in Germany the "German Democratic Republic" in 1949. In the early 1950s, the US worked for the rearmament of West Germany and, in 1955, its full membership to NATO.
In 1949 Mao's Red Army defeated the US-backed Kuomintang regime in China. Shortly afterwards, the Soviet Union created an alliance with the newly formed People's Republic of China. Confronted with the Chinese Revolution and the end of the US atomic monopoly in 1949, the Truman administration quickly moved to escalate and expand the containment policy. In a secret 1950 document, NSC-68, Truman administration officials proposed to reinforce pro-Western alliance systems and quadruple spending on defence.
US officials moved thereafter to expand "containment" into Asia, Africa, and Latin America. At the same time, revolutionary nationalist movements, often led by Communist parties, were fighting against the restoration of Europe's colonial empires in South-East Asia. The US formalized an alliance with Japan in the early 1950s, thereby guaranteeing the United States a number of long-term military bases. Truman also brought other states, including Australia, New Zealand, Thailand and the Philippines, into a series of alliances.
To Stalin's surprise, Truman committed US forces to drive back the North Koreans. Public opinion in countries such as Great Britain, usual allies of the US, was divided for and against the war. British Attorney General Sir Hartley Shawcross repudiated the sentiment of those opposed when he said "I know there are some who think that the horror and devastation of a world war now would be so frightful, whoever won, and the damage to civilization so lasting, that it would be better to submit to Communist domination. I understand that view - but I reject it.  In 1953, the Korean War ended in stalemate, but the US gradually got itself entangled in another civil war. The US supported the South Vietnamese government against North Vietnam, which was backed by the Soviet Union and China.