How and why were the cold war geographic boundaries drawn?

He Cold War is the post-World War II period, from 1945 to 1991, which culminates with the fall of the Soviet Union. It is called the “Cold War” because instead of military confrontation, it was an ideological war of influence, between the capitalist bloc, led by United States, and the communist bloc, led by the Soviet Union –today’s Russia (formally the Union of Soviet Socialist Republics, which existed from 1922 to 1991, a federation of 15 republics dominated by Russia). Sometimes, during this period, the whole world felt the tension of an approaching third world war where nuclear weapons would be used. A consequence of this genuine anxiety was the arms race, where the superpowers competed in the development of more powerful weapons of mass destruction. The more destructive capacity a country had the more political influence it would gain. Some outstanding events are remembered: the witch hunt of McCarthyism (1950-1956), when Republican Senator Joseph McCarthy of Wisconsin accused thousands of Americans of being Communists and summoned them for hearings at the House Un-American Activities Committee in Congress; the wrath of Soviet President Nikita Khrushchev at the United Nations in 1960, when he yanked off his shoe and thumped on his desk over a debate on colonialism; the Cuban missile crisis in 1963, the policy of Détente in 1971, Perestroika in the 1980’s, and the fall of the Berlin Wall and the German reunification in 1990. 
As the Cold war chill spread, US President Harry Truman initiated the “Truman Doctrine” and offered military and economic aid to countries threatened by a Communist takeover. The idea was to “contain” the spread of communism. A key part of the Truman Doctrine was the “Marshall Plan”, which gave massive economic aid to European nations to rebuild their economies destroyed by the war. In 1949, the North Atlantic Treaty Organization (NATO) was set up. NATO was a military alliance of US, Canada and Western Europe against the Soviet Union and the Eastern Bloc. The Soviets created the Warsaw Pact as a mutual defense treaty between eight communist states and it was a counter-alliance to NATO. Thousands of soldiers representing the two ideologies backed by the superpowers faced each other along a line dividing East and West. 
In 1985, a new leader, Mikhail Gorbachev, was appointed head of state of the Soviet Union. He shared the views of the reformers who recognized the fundamental economic problems facing the Soviet Union. In 1985 he embarked on a three-pronged policy: perestroika (restructuring), glasnost (openness) and military disarmament. Under perestroika, planning was decentralized, allowing market forces to work. Agriculture and land was put in the hands of families and cooperatives rather than large state farms. The results were a mixed economy featuring a blend of socialist planning and capitalist free market. At the same time, arms reduction talks were initiated with United States. Both sides agreed to limit nuclear weapons. To get political backing for his reforms, Gorbachev introduced glasnost. Censorship was curtailed, encouraging free discussion of everything from culture to politics. Partial democratization of the Communist Party and the Soviet political system followed. In the spring of 1989, the first open elections since 1917 were held, resulting in the defeat of numerous communist candidates. In 1990, after Gorbachev, President Boris Yeltsin resigned from the Communist Party and declared Russia an independent republic. In 1991, Ukraine, Armenia, Georgia and other former Soviet republics did the same. The Cold War ended.

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