What Communist country rose to international power after World War II?

The United States and the Soviet Union had been on the same side during World War II, members together of the Allies fighting against the Axis Powers. Following the war, however, the USA and USSR became heated rivals in what came to be known as the Cold War.

Immediate conflicts that caused tension between the USA and the USSR as opposing international powers:

The USA had atomic weapons and the USSR did not. The US would not share that technology with the Soviets. However, the Soviets developed their own atomic bomb by 1949, and this brought about a huge arms race between the two powers.
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 of the tension between the two leading international powers were:

The USA was committed to capitalism and democratic institutions of government.
The USSR was committed to communism and imposed authoritarian government.

The Soviet Union became a leading international power as the leading Communist nation, aligned against Western democracies.