Immediate conflicts: ... The USA had atomic weapons and the USSR did not. (The US would not share that technology with the Soviets.) ... The USSR did not assure that free and fair elections took place in Eastern Europe -- it saw to it that Soviet-aligned governments were installed there. ... Tensions over the East Germany / West Germany and East Berlin / West Berlin division of territory.
Deeper causes: 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.
Britain had expected the Southern colonies to remain loyal, in part because of their reliance on slavery. Many in the Southern colonies feared that a rebellion against the mother country would also trigger a slave uprising against the planters.
It was a series of programs that ensured workers during the great depression, that Roosevelt would restore prosperity to them and their country. He stabilized the economy to provide those with jobs who were suffering.
