The United States had invited Britain into its atomic bomb project but kept it secret from the Soviet Union. Stalin was aware that the Americans were working on the atomic bomb, and he reacted to the news calmly. One week after the end of the Potsdam Conference, the US bombed Hiroshima and Nagasaki. Shortly after the attacks, Stalin protested to US officials when Truman offered the Soviets little real influence in occupied Japan. Stalin was also outraged by the actual dropping of the bombs, calling them a "superbarbarity" and claiming that "the balance has been destroyed...That cannot be." The Truman administration intended to use its ongoing nuclear weapons program to pressure the Soviet Union in international relations.

Following the war, the United States and the United Kingdom used military forces in Greece and Korea to remove indigenous governments and forces seen as communist.

During the opening stages of World War II, the Soviet Union laid the foundation for the Eastern Bloc by invading and then annexing several countries as Soviet Socialist Republics, by agreement with Germany in the Molotov–Ribbentrop Pact. These included eastern Poland, Latvia, Estonia, Lithuania, part of eastern Finland and eastern Romania. Central and Eastern European territories that the Soviet army liberated from Germany were added to the Eastern Bloc, pursuant to the percentages agreement between Churchill and Stalin, which, however, contain provisions regarding neither Poland nor Czechoslovakia or Germany.