Even though the Soviet Union was an ally, we never fully trusted them during WWII. The German's miscalculation regarding how difficult it would be to invade Russia gave the Soviets resolve and we reluctantly came to their aid. As part of the Marshall Plan, we allowed them to cut up Europe and present communism to a large part of the world, even though the Plan was designed to not only assist Europe financially, but to also eliminate communism in the region. The Soviet Union even rejected monies to assist them after WWII. Their unyielding attitude and our lack of action assisted the Cold War along. Not to mention the Korean War, where Soviet presence was felt during the entire campaign.
World War II set the stage for the Cold War in at least two ways. First, the war featured the Soviet Union being invaded and almost destroyed from the west. This renewed Russian fears of attack from that direction and made them want a buffer zone. Second, at the end of the war, the Soviets were able to take much of Eastern Europe as well as Northern Korea. By overrunning these areas the Soviet Union took what would become its satellite states. By taking and dominating those countries, the Soviets provided what the West saw as proof of its desire to dominate the world. This was the immediate origin of the Cold War.