1. German army overestimated the support they believed they would recieve from the Soviet people, they assumed that hate for the existing soviet regime would mean that people supported the German invasion. Assumption was not completely unjustified - people in Ukraine and parts of Bellarus did support the Germans upon the initial invasion but this was ruined by German atrocities and extreme violence.
2. German soldiers committed extreme acts of violence against the Soviet people. Village account said that "they hanged the womnan teacher and the chairman and ***** the girls. It was this extreme violence that may have ruined the initial support from the Soviet people. But its possible that these acts of violence were committed by other groups following through after the Germans like the SS.
3. The German armys were extremely unprepared. They assumed that the war would be won and over before the beginning of winter. Meaning that they had no real winter clothing and the cold weather hit them hard. Soldiers were fighting in -40c temperatures.
1. Propaganda. Stalin with held numbers of soviet losses from the people possibly to keep the morale up. He also used posters and other propaganda methods that presented himself almost as a god, enticing people to put all their faith into him. Finally, the war was referred to as "The Great Patriotic War" intentially to imbue the people and convince them to fight for their country.
2. He was fierce and brutal to his generals. An example would be not allowing them to retreat from East fom Moscow, he contiually painted himself as the one and only leader. However he sometimes made disasterous decisions - his refusal to allow retreat from Kharkov in 1943 led to its's encirclement and destruction.
3. Stalin also made mistakes too. Upon the initial invasion he refused to believe there was a problem or accept that Hitler would break the Kellogg-Briand Peace Pact. He retreated to his countryside home for 10 days before he gave any kind of instruction. Some hsitorians say that his retreat was merely a test of loyalty for his generals rather than some form of breakdown, backed up by the knowledge that Stalin was an extremely paranoid man.
1. The weather. Harsh condiditons hit the Nazi's hard due to them being underprepared whereas the USSR had learned from their battle with Finland of the importance of being prepared for winter and had fur caps, camoflauge boots and warm felt coats. They also had entire battalions trained to fight on skiis and superior equiptment. Heinz Guderian lost twice as many men from frost as from enemy action. However, some historians decide that this was not a deciding factor to the victory of the war.
2. Allied victory. The USSR were provided with trucks and jeeps in the 1942 Lend-Lease scheme from America. Some say it may have accounted for up to a fifth of Soviet resources and that the USSR would never have won the war without the Lend-Lease scheme. Although other people (Alan Clark, Barbarossa 1965) said that the Soviets could have fought the Germans to a standstill without foreign aid.
3. Soviet People. People fought selflessly because of a feeling of partiotism regardless of their hate for the regime. This factor is closely linked to Stalins leadership and his ability to evoke passion in his people.