-Stalin didn't keep the agreements he had made with the Western Allies about holding free elections after the war in countries occupied by the Soviet Army.
-This scared the West, who believed that Stalin would try to take over the world by force - Communist ideology clearly stated Communism would inevitably take over the entire world eventually, by force if necessary
-The Americans didn't tell Stalin about the Manhattan Project, the building of the A- bomb. After using A-bombs successfully against Japan, USA refused to share the technology with USSR.
-This scared the Russians, who thought USA may start a new war, against them, using A-bombs.