In 1939 Germany and the Soviet Union had signed a non-aggression pact, which secretly promised Russia half of Poland, Bessarabia and the Baltic States. Germany had promised this because they were afraid of an alliance between Britain, France and the USSR. During 1940 the Soviets invaded the Baltic States and Bessarabia. This brought them close to the Romanian oil fields, which were the Germans main source of oil. They then began to put pressure on Finland, Germany's chief source of nickel. At this time the German high command began thinking of attacking Russia to protect its resources and ordered the build up of forces along the Eastern Front. By 1941 Germany had already successfully invaded France, Norway, Denmark, Belgium, Holland and Poland and was engaged with British forces in North Africa. America had already taken military action against Germany when it dropped depth charges on a U-boat in the North Atlantic on the 10th of April. Hitler was concerned that Britain and the United States might form an alliance with USSR and decided that the time was right to invade. .
The German strategy was to simultaneously attack Leningrad, Moscow and Smolensk and they began their attack with a massive artillery bombardment which caught the Russians by surprise and within the first 48 hours of the campaign the Luftwaffe had destroyed 2000 Russian aircraft, handing air superiority over to the Germans. Three Army Groups were put in place. Army Group North, which would advance to Leningrad, Army Group Center, which would advance to Smolensk and Army Group South which was ordered to push towards Ukraine. These initial German advances were a success. By early July, the panzers from the North divisions were within 80 miles of Leningrad and the central forces had taken Smolensk. In mid-July leadership debates over strategy arose. Hitler favored a concentrated attack on Leningrad and Ukraine but with the fall of Smolensk his generals presumed that Moscow would become the main objective.
