The underlying causes of the Cold War came from dramatically different worldviews held by the United States of America and the Union of Soviet Socialist Republics. The USA was committed to capitalism and democratic institutions of government. The USSR was committed to communism and imposed authoritarian government. The Cold War was mostly a tension between these worldviews.
There also were immediate issues in the aftermath of World War II that drove the USA and USSR from being allies to being rivals. The USA had atomic weapons and the USSR did not. (The US would not share that technology with the Soviets.) By 1949, the USSR developed and tested the first of its own atomic weapons, and the Cold War rivalry became a nuclear rivalry. The two countries were also at odds over how governments in Europe would be reconstituted after World War II. The USA wanted free and fair elections and democratic countries to develop, whereas the Soviet Union was looking for governments in Eastern Europe that would align with its communist system and provide a security buffer against what it saw as capitalist imperialism.
Many people know this as the "corrupt bargain". Essentially the election was between three candidates: John Quincy Adams, Andrew Jackson, and Henry Clay (who was Speaker of the House at the time). Although Andrew Jackson won the popular vote, none of the candidates met the constitutional requirement for victory. So the House of Representatives had to decide who was president. For some reason Henry Clay was automatically eliminated leaving only John Quincy Adams and Andrew Jackson as candidates. Henry Clay had previous discussions with John Quincy Adams. It is unknown if they struck a deal. But Henry Clay said he supported John Quincy Adams. And since he had a lot of influence in the House John Quincy Adams won. Also Henry Clay became Secretary of State. Furious Andrew Jackson called it the "corrupt bargain".
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