The Yalta Conference took place in February, 1945. Two months later Roosevelt died and Truman became President. Within a week of Roosevelt's funeral, Truman had begun to reverse Roosevelt's foreign policy. Instead of American-Soviet cooperation, Truman established a "get tough" policy with Russia. Truman's flip-flop became apparent during a visit of Foreign Minister Molotov to the United States to attend the United Nations San Francisco Conference.
This article can also be found at the Monthly Review website, where most recent articles are published in full.