Harry S. Truman entered the White House in April 1945 expecting to continue Roosevelt's effort to build a collaborative relationship with the Soviet Union. Yet, within a year, the Grand Alliance was in shreds and the United States and the Soviet Union had again become enemies. Primarily because of a drastic intensification of the Cold War during Truman's presidency, the United States would abandon its prewar isolationism once and for all and adopt a policy of containing the expansion of communism in Europe and the Far East. Historians have argued about the reasons for the breakdown of the Grand Alliance ever since.
Truman was at least partially responsible for the postwar breakdown of Soviet-American relations. With almost no experience in international relations, the new president was much more susceptible to the anti-Soviet views of former Roosevelt advisers who stayed on in Truman's administration, particularly Admiral William Leahy, the military chief of staff, Secretary of the Navy James V. Forrestal, and Moscow ambassador Averell Harriman, than the deceased president had been. Along with Winston Churchill, they pressed Truman to take a tougher stance against the Soviets.
At first Truman complied. On April 16 he joined Churchill in sending a message to Stalin insisting that the Soviets abide by the Yalta accord on Poland. In a White House meeting on April 23, Truman personally berated Soviet Foreign Minister Molotov for failing to observe that agreement. On