What actions of Stalin began the Cold War?

Stalin's decision to install a Communist government in Poland is perhaps the single event that, from the perspective of the United States, at least, can be said to have begun the Cold War. The Soviet leader had already agreed, with Franklin Roosevelt at the Yalta Conference a few months before, to allow for democratic elections in Poland. However, with Poland occupied by Soviet troops at the end of the war in Europe, he changed his mind, and set up a communist state in that country, despite his private misgivings about the feasibility of such a government in Poland. The Potsdam Conference of July 1945 took place with this development in the background, and Harry Truman, having assumed the office upon the death of Franklin Roosevelt in April of that year, was incensed. He believed that the Soviets had betrayed their promise at Yalta, and saw Soviet influence in Poland as an act of aggression. This event illustrated that cracks were already beginning to develop in the never-comfortable alliance between the United States and the Soviet Union. It should be noted, however, that some historians question the extent to which Stalin was responsible for beginning the Cold War. Some, for example,  point to the use of the atomic bomb on Japan as the single event that began the conflict, which played out against the backdrop of the threat posed by atomic and later nuclear weapons.