Prelude to War
When German troops invaded Poland, the Soviet Union was ill prepared to fight a major war. Although military expenditures had increased dramatically during the 1930s and the standing army was expanded in 1939, Soviet weaponry was inferior to that of the German army. More important, eight of the nation's top military leaders, including Marshal Mikhail Tukhachevskiy, had been executed in 1937 in the course of Stalin's purges; thus the armed forces' morale and effectiveness were diminished. The time gained through the pact with the Nazis was therefore critical to the recovery of Soviet defenses, particularly because Hitler's forces had overrun much of Western Europe by the summer of 1940. To strengthen its western frontier, the Soviet Union quickly secured the territory located in its sphere of interest. Soviet forces seized eastern Poland in September 1939; entered Estonia, Latvia, and Lithuania in October 1939; and seized the Romanian territories of Bessarabia (later incorporated into the Moldavian Republic) and northern Bukovina (later added to the Ukrainian Republic) in June 1940. Only Finland resisted Stalin's program of expansion, first by refusing to cede territory and then by putting up a determined defense along the Mannerheim Line when the Red Army invaded in November 1939. The Soviet-Finnish War (also known as the Winter War) of 1939-40 exposed grave deficiencies in Soviet military capabilities, which Hitler undoubtedly noted.
As the European war continued and the theaters of the conflict widened, Hitler began to chafe under his pact with the Soviet Union. The German dictator refused to grant Stalin a free hand in the Balkans, instead moving the German forces deeper into Eastern Europe and strengthening his ties with Finland. Hitler thus prepared for war against the Soviet Union under a plan that he officially approved in December 1940. At this point, however, Stalin still apparently believed that the Soviet Union could avert war by appeasing Germany. To achieve this goal, regular shipments of Soviet materials to Germany continued, and the Soviet armed forces were kept at a low stage of readiness. But despite Stalin's efforts to mollify Hitler, Germany declared war on the Soviet Union just as 180 German divisions swept across the border early on the morning of June 22, 1941.