• In 1919 the former World War I allies of Britain, France and the United States joined the "White Russians" to fight off the Bolsheviks following the revolution.  (For more information see The American Invasion of Russia). Although this intervention failed and the Red Army of the Bolsheviks secured the power of the new Soviet state, the young USSR government never quite trusted the western democracies after that.
• The western democracies did not invite the Soviet Union to participate in the World War I peace talks or the League of Nations.
• The west did not aid the Republicans fighting the fascists in the Spanish Civil War.
• The west did not invite the Soviets to the Munich Conference which decided the fate of Czechoslovakia in the years leading up to World War II, even though the Soviet Union had a security pact with Czechoslovakia.

The west, for its part, never trusted the Soviet Union:

• The avowed purpose of the International Communist Party to secure world wide communist revolution. There was a great fear of socialism in Europe and America.
• The Soviets negotiated an agreement with Hitler and annexed eastern Poland.
• By the end of the war Britain and the United States distrusted the Soviet motives in eastern Europe.

This mutual distrust was barely suppressed during World War II when for practical reasons (the common enemy of Hitler's Germany) the western allies and the Soviet Union became uneasy allies. 

Stalin believed that the western allies were dragging their feet in opening up the "second front" in Europe, so necessary to take the pressure off the struggling Soviet forces in the east.

Stalin was open about wanting "friendly governments" in Eastern Europe to protect his country's western frontier from another invasion like the invasion so recently experienced by Germany.

All of this was in the air when Stalin, Churcill and Roosevelt met at the end of World War II.