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 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.
