It is difficult to say that any one person was responsible. The first person who comes to mind is Joseph Stalin, who commented at the end of World War II that the war against Fascism had ended; the war against Capitalism had begun. Stalin refused to withdraw troops from Eastern Europe and even attempted to starve out West Berlin in an attempt to take all of that city. It was only the Berlin Airlift that stopped it. Stalin's continued threats of aggression, and the Soviet's development of an Atomic Weapon led the United States to counter that development by building more arms itself. The end result was the policy of "brinksmanship," that is, going to the brink of nuclear war. For the first time in our history, the phrase "mutually assured destruction" came into vogue.
Stalin cannot bear the burden alone, however. At the Yalta Conference, Franklin Roosevelt, who was very sick, dying in fact, made a number of concessions to Stalin that he never should have made. He was intent on getting a committment from Stalin to enter the war against Japan, and believed--wrongly it turned out--that he could trust Stalin. Also, some blame must be laid at the feet of General Dwight D. Eisenhower. When the Soviets and Americans were closing in on Berlin, Eisenhower elected not to take Berlin, but let the Soviets take it instead. He said it was not strategically important. Later when he was President, he came to rue that decision.