Soviet foreign policy, at first friendly toward Germany and antagonistic toward Britain and France and then, after Hitler's rise to power in 1933, becoming anti-Fascist and pro-League of Nations, took an abrupt turn on Aug. 24, 1939, with the signing of a nonaggression pact with Nazi Germany. The next month, Moscow joined in the German attack on Poland, seizing territory later incorporated into the Ukrainian and Belarussian S.S.R.'s. The war with Finland (1939-40) added territory to the Karelian S.S.R. set up March 31, 1940; the annexation of Bessarabia and Bukovina from Romania became part of the new Moldavian S.S.R. on Aug. 2, 1940; and the annexation of the Baltic republics of Estonia, Latvia, and Lithuania in June 1940 created the 14th, 15th, and 16th Soviet Republics. The illegal annexation of the Baltic republics was never recognized by the U.S. for the 51 years leading up to Soviet recognition of Estonia, Latvia, and Lithuania's independence on Sept. 6, 1991. The Soviet-German collaboration ended abruptly with a lightning attack by Hitler on June 22, 1941, which seized 500,000 square miles of Russian territory before Soviet defenses, aided by U.S. and British arms, could halt it. The Soviet resurgence at Stalingrad from Nov. 1942 to Feb. 1943 marked the turning point in a long battle, ending in the final offensive of Jan. 1945. Then, after denouncing a 1941 nonaggression pact with Japan in April 1945, when Allied forces were nearing victory in the Pacific, the Soviet Union declared war on Japan on Aug. 8, 1945, and quickly occupied Manchuria, Karafuto, and the Kuril islands.

The U.S.S.R. built a cordon of Communist states running from Poland in the north to Albania and Bulgaria in the south, including East Germany, Czechoslovakia, Hungary, and Romania, composed of the territories Soviet troops occupied at the war's end. With its Eastern front solidified, the Soviet Union launched a political offensive against the non-Communist West, moving first to block the Western access to Berlin. The Western powers countered with an airlift, completed unification of West Germany, and organized the defense of Western Europe in the North Atlantic Treaty Organization (NATO). Stalin died on March 6, 1953, and was succeeded the next day by G. M. Malenkov as premier.