While some historians have suggested that the decision to drop the atomic bomb on Japan was motivated by Truman's desire to check the Soviets, this is a somewhat controversial notion. For this answer we will stick to more concrete facts. Truman came to believe that it was in the best interests of the United States to stop the spread of communism and the influence of the Soviet Union. At the Potsdam Conference in the summer of 1945, differences in how postwar Europe surfaced between Truman and Stalin. This is particularly true of the question of the Polish border and the shape that postwar Germany would take. After the war, Truman adopted a policy of containment that would dominate American foreign policy for the better part of four decades. The policy stated that the United States had an interest in stopping the spread of communism to new areas. The Truman Doctrine and Marshall Plan were also American financial commitments to aid economically distressed countries in an effort to stop the spread of communism. Harry Truman put the containment policy into action when he committed troops to South Korea in response to communist aggression on that peninsula.