The Soviet Union and the U.S. were on opposite ends when it came to politics. The U.S.'s democracy was thriving, while the USSR was communist and lead by a totalitarian leader.
The U.S. feared that communism would spread, especially since Stalin was promoting it around the world. The U.S. in turn created the Truman Doctrine, which offered aid to any country that was falling into communism. This made Stalin's influence decrease.
America joined this alliance along with many other countries. A few years later, the USSR had it's Eastern allies form an alliance called the Warsaw Pact. This caused tensions to rise between the two countries.
The U.S. and the Soviet Union had serious competition between each other. each competed to be the first to launch a successful mission in space, having the strongest military, and spreading their political views across the world.
Stalin: Communism shall spread everywhere!!!!!
The Marshall Plan, made by the U.S. to stop the spread of communism throughout Europe, caused a great amount of tension. The Soviet Union had already created influence in Eastern Europe where it bordered. This created a Communist bloc called the Iron Curtain, where Eastern and Western Europe were blocked off from each other in terms of immigration, trade, and diplomacy.