The Soviet Union and the United States slowly started to show their distrust for the other. Some of the ways this presented itself included:
Big 3 Alliance:
During their Second World War alliance, Britain and the United States had been closer friends than either was with the Soviet Union. The Soviet Union had asked the United States and the Allies to start a second front during the War, which they did with D-Day in 1944. Stalin, however, felt that they waited longer than necessary so that the Soviet Union would be weakened by Germany. This planted seeds of distrust.
The Atomic Bomb:
The Soviet Union was not included in the American/British creation of the atomic bomb during the Second World War. They felt they were doing their part in the alliance and did not like being left out of something so important. This helped to further cement distrust.
The Marshall Plan:
This distrust was further stretched when the United States created the Marshall Plan in 1947 to help rebuild Europe. The United States would provide assistance to any country as long as they closed off any large communist movement in their country.
Needless to say, Stalin was against it and thought it would just encourage capitalism to prosper in these places, so he and the other communist countries did not accept. The Soviet Union would later create the Council for Mutual Economic Assistance (COMECON) in 1948 instead, redrawing Eastern Europe’s economies so that they followed that of the Soviet Union.