What is the traditional interpretation of why the cold war started?

My understanding of The Cold War was that it was a series of non-violent competitions between the Soviet Union and the United States where they basically wanted to see who was better. In summary, the Cold War began in 1945 and was a period of tension between the United States and Soviet Union. President Harry Truman fueled the fire by using the atomic bomb as leverage and issuing the Truman Doctrine to fund democracy in third world nations.