What was the first event of the Cold War?

Most people would agree that the US choosing to keep atomic energy and the development of the atomic bomb a secret from Joseph Stalin in 1940-1945 was the event that began the Cold War. The Cold War was really nothing more than the US and the USSR trying to reach a "balance of terror," or a point where each country was sure that the other would not attack them for fear of the reprimanding attack. It began when Roosevelt bombed Hiroshima and Nagasaki. Stalin saw that the US had highly superior weapons, and was afraid they would use them againt the USSR, and so he began developing nuclear weapons of his own.