Why did tensions between these two allies ( us and soviet union) arise ?

Relations between the Soviet Union and the United States were driven by a complex interplay of ideological, political, and economic factors, which led to shifts between cautious cooperation and often bitter superpower rivalry over the years. The distinct differences in the political systems of the two countries often prevented them from reaching a mutual understanding on key policy issues and even, as in the case of the Cuban missile crisis, brought them to the brink of war.

The United States government was initially hostile to the Soviet leaders for taking Russia out of World War I and was opposed to a state ideologically based on communism. Although the United States embarked on a famine relief program in the Soviet Union in the early 1920s and American businessmen established commercial ties there during the period of the New Economic Policy (1921–29), the two countries did not establish diplomatic relations until 1933. By that time, the totalitarian nature of Joseph Stalin's regime presented an insurmountable obstacle to friendly relations with the West. Although World War II brought the two countries into alliance, based on the common aim of defeating Nazi Germany, the Soviet Union's aggressive, antidemocratic policy toward Eastern Europe had created tensions even before the war ended.

The Soviet Union and the United States stayed far apart during the next three decades of superpower conflict and the nuclear and missile arms race. Beginning in the early 1970s, the Soviet regime proclaimed a policy of détente and sought increased economic cooperation and disarmament negotiations with the West. However, the Soviet stance on human rights and its invasion of Afghanistan in 1979 created new tensions between the two countries. These tensions continued to exist until the dramatic democratic changes of 1989–91 led to the collapse during this past year of the Communist system and opened the way for an unprecedented new friendship between the United States and Russia, as well as the other new nations of the former Soviet Union.

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