After the Axis Powers were defeated, the differences between the United States and the Soviet Union came to the front.Having freed Eastern Europe from the Nazis, the Soviet army stayed in the conquered areas and set up pro-Soviet regimes in Poland, Romania, Bulgaria, and Hungary.As Dean Acheson, U.S. undersecretary of state, explained on February 29, 1947: PRIMARY SOURCE

Like apples in a barrel infected by one rotten one, the corruption of Greece would

Infect Iran and all to the east.Truman of the United States, alarmed by British weakness and the possibility of Soviet expansion into the eastern Mediterranean, responded with the Truman Doctrine.

After the Axis Powers were defeated, the differences between the United States and the Soviet Union came to the front. Stalin still feared the capitalist West, and Western leaders still had a great fear of communism. It should not surprise us that two such different systems would come into conflict.
Because of its need to feel secure on its western border, the Soviet Union was not prepared to give up its control of Eastern Europe after Germany's defeat. Nor were American leaders willing to give up the power and prestige the United States had gained throughout the world. Suspicious of each other's motives, the United States and the Soviet Union soon became rivals.
Eastern Europe was the first area of disagreement. The United States and Great Britain believed that the liberated nations of Eastern Europe should freely determine their own governments.
Stalin, however, fearful that these nations would be anti-Soviet if they were permitted to have free elections, opposed the West's plans. Having freed Eastern Europe from the Nazis, the Soviet army stayed in the conquered areas and set up pro-Soviet regimes in Poland, Romania, Bulgaria, and Hungary.
A civil war in Greece created another area of conflict between the superpowers. The Communist People's Liberation Army and anti-communist forces supported by Great Britain fought for control of Greece in 1946. However, Britain had its own economic problems, which caused it to withdraw its aid from Greece.

The Truman Doctrine and the Marshall Plan
President Harry 5. Truman of the United States, alarmed by British weakness and the possibility of Soviet expansion into the eastern Mediterranean, responded with the Truman Doctrine. This doctrine, created in 1947, said that the United States would provide money to countries (in this case, Greece and Turkey) that were threatened by communist expansion. If the Soviet Union was not stopped in Greece and Turkey, the Truman argument ran, then the United States would have to face the spread of communism throughout the free world. As Dean Acheson, U.S. undersecretary of state, explained on February 29, 1947:
PRIMARY SOURCE

Like apples in a barrel infected by one rotten one, the corruption of Greece would

Infect Iran and all to the east. It would also carry infection to Africa through Asia Minor and Egypt, and to Europe ..."

• from Present at the Creation: My Years in the State Department
  The Truman Doctrine was soon followed by the European Recovery Program. Proposed in June 1947 by General George C. Marshall, U.S. secretary of state, it is better known as the Marshall Plan. Marshall believed that communism was successful only in countries that had economic problems. Thus, to prevent the spread of communism, the Marshall Plan provided close to $13 billion to rebuild war-torn Europe.
  The Marshall Plan did not intend to exclude the Soviet Union or its economically and politically dependent Eastern European satellite states. Those states refused to participate, however. According to the Soviet view, the Marshall Plan guaranteed
  "American loans in return for the relinquishing by the European states of their economic and later also their political independence." The Soviets saw the
  FIRST CARGO OF CARIBBEAN SUGAR
  SHIPPED UNDER MARSHALL
  AID
  Marshall Plan as an attempt to buy the support of the smaller European countries.
  In 1949 the Soviet Union responded to the Marshall Plan by founding the Council for Mutual Economic Assistance (COMECON) for the economic cooperation of the Eastern European states. COMECON largely failed, however, because the Soviet Union was unable to provide much financial aid.
  By 1947, the split in Europe between the United States and the Soviet Union had become a fact of life. In July 1947, George Kennan, a well-known U.S. diplomat with much knowledge of Soviet affairs, argued for a policy of containment to keep communism within its existing boundaries and to prevent further Soviet aggressive moves. Containment of the Soviet Union became formal U.S. policy.
  The Division of Germany and the Berlin Airlift
  The fate of Germany also became a source of heated contention between the Soviets and the West. At the end of World War Il, the Allied Powers had divided Germany into four zones, each occupied by one of the Allies-the United States, the Soviet Union, Great Britain, and France. The city of Berlin, located deep inside the Soviet zone, was also divided into four zones.

The foreign ministers of the four occupying powers mehra platedly in an attempt to arrive at a final peace treaty with Germany but had little succes At the same time, Great Britain, France, and the United States gradually began to merge their zones ecanomically, By February 1948, Great Britain, France, and the United States were making plans to unify the three Western sections of Germany (and Berlin) and create a West German government.
The Soviets reacted with a blockade of West Berlin, which allowed neither trucks, nor trains, nor barges to enter the city's three Western zones, Food and supplies could no longer get through to the 2.5 million people in these zones. The Russians hoped to secure economic control of all Berlin and force the Western powers to halt the creation of a separate West German state,
The Western powers faced a dilemma. No one wanted to risk World War III. Therefore, an attempt to break through the Soviet blockade with tanks and trucks was ruled out. However, how could the people in the Western zones of Berlin be kept alive when the whole city was blockaded inside the Soviet zone? The solution was the Berlin Airlift-supplies would be flown in by American and British airplanes. For more than 10 months, more than 200,000 flights carried 2.3 million tons (1.4 million t) of supplies.
At the height of the Berlin Airlift, 13,000 tons (11,800 t) of supplies were flown daily to Berlin. The Soviets, also not wanting war, finally gave in and lifted the blockade in May 1949.
The blockade of Berlin increased tensions between the United States and the Soviet Union. It also brought the separation of Germany into two states. In September 1949, the Federal Republic of Germany, or West Germany, was formally created. Its capital was Bonn. Less than a month later, a separate East German state, the German Democratic Republic, was set up by the Soviets. East Berlin became its capital. Berlin was now divided into two parts, a reminder of the division between West and East.