A lot of people forget that the Soviet Union did about 90% of the actual fighting in the war against Hitler. Starting in 1942 Stalin repeatedly asked his "allies" in the west to either open a second front Europe or at least send military forces, particularly air power to help Soviet forces. In both cases he got promises that were never kept.
What he got instead was a declaration of a policy of "unconditional surrender" in early 1943 about which he was never consulted, a policy which gave the Germans every incentive to fight to the bitter end. In spite of repeated protests from Stalin to modify this ill-considered policy, Roosevelt flatly refused to change his position, guaranteeing continued huge Russian losses while the west postponed its promised attack in the west for two years.
Even before the onset of the Cold War in the years after 1945, tension and distrust marked the relationship between Great Britain, the United States and the Soviet Union. Although this tension was understandable given the ideological differences between capitalism and communism, a more important immediate cause was the bizarre, even duplicitous, conduct of President Franklin D. Roosevelt toward Josef Stalin.
The evidence below concerns two important areas of disagreement between the Soviet union and the Western Allies, especially the United States, during the Second World War: 1) the opening of a second front against Germany in Western Europe and 2) the policy of unconditional surrender. Unfulfilled promises made to Stalin by Roosevelt and Churchill (the former in particular) about opening a second front in 1942 and then the American-British adoption of a policy of unconditional surrender without consulting the Soviets deepened the distrust between the West and the Soviets that later developed into the Cold War.