Despite those setbacks, to many, World War II seemed like it was over within a year: Germany controlled Poland, Norway, the Netherlands, Denmark, France, and Belgium, all within nine months of the initial attack on Poland. As noted above, its forces were soon making headway in the Balkans and North Africa as well. Hitler had first conceived of the war against the USSR as something to be accomplished after defeating the rest of Europe, and thus the planned invasion of Britain was to be the final step before the Soviet invasion. The fact that Britain was not only holding out, but holding on, however, led to a change in German plans: the Soviet invasion would have to occur before Britain was defeated.
In the overall context of the war, by far the largest and most important target for Germany was the Soviet Union. The non-aggression pact signed just before the beginning of the war between the USSR and Germany had given the Nazis the time to concentrate on subduing the rest of Europe. By the spring of 1941, Hitler felt confident that an all-out attack on the USSR was certain to succeed, now that German military resources could be concentrated in the east. He was spurred on by the fact that, according to his own racial ideology, the Slavs of Eastern Europe (most obviously the Russians) were so inferior to the "Aryan" Germans that they would be unable to mount an effective resistance. Thus, Hitler anticipated the conquest of the Soviet Union taking about ten weeks.
For his part, Stalin did not think Hitler would be foolish enough to try to invade Soviet Union, especially before Germany had truly “won” in the west. In 1939, Stalin reported to his advisers that “The war will be fought between two groups of capitalist states…we have nothing against it if they batter and weaken each other. It would be no bad thing if Germany were to knock the richest capitalist countries (particularly England) off their feet.” Furthermore, every European school child learned about Napoleon’s disastrous attempted invasion of Russia in 1812, and thus the sheer size of Soviet territory seemed like a logical impediment to invasion.
While we now know that he was completely wrong about Hitler’s intentions, Stalin had good reason for not thinking that Germany would dare attack - the USSR had one-sixth of the land surface of the earth, with a population of about 170,000,000. Its standing army as of 1941 was 5.5 million strong, with 12 million in reserve. It also had a vast superiority in quantity (albeit not quality) of equipment at the start of the war. Indeed, by the end of the war, the Soviets had mobilized 30.6 million soldiers (of whom 800,000 were women: the USSR was the only nation to rely on women in front-line combat roles, at which they equaled their male countrymen in effectiveness). Given that vast strength, Stalin was astonished when the Germans attacked, reportedly spending hours in a daze before ordering an armed response.
In June of 1941, Germany invaded the USSR with over 3 million troops. This invasion was codenamed Operation Barbarossa, after a medieval German king who warred with the Slavs. The first few months were a horrendous disaster for the Soviets. The Soviet air force was utterly destroyed, as were most of its armored divisions. Hundreds of thousands of Soviet soldiers were taken prisoner. Josef Stalin had spent the late 1930s "purging" various groups within the Soviet state and the army, and his purges had already killed almost all of the experienced commanders, leaving inexperienced and sometimes inept replacements in their wake. In many areas, the locals actually welcomed the Germans as a better controlling force than the Bolsheviks had been, putting up no resistance at all. Even though Hitler himself was frustrated to discover than his ten-week estimate of conquest was inaccurate, the first months of the invasion still amounted to an astonishing success for German forces.
Despite its early success, however, the German advance halted by winter. The initial welcome German soldiers received vanished when it was revealed that the German army and the Nazi SS were at least as bad as had been the communists, pressing people into work gangs, murdering resisters, and most importantly, shipping everything that could possibly be useful for the German war effort back to Germany, including both equipment and foodstuffs. Thus, groups of “partisans” (i.e. insurgents) mounted successful resistance movements that cost the Germans men and resources. Likewise, German forces had advanced so quickly that they were often bogged down in transit, with German supply lines stretched to the breaking point. Thus, just as had happened during Napoleon's retreat over a hundred years earlier, guerrilla fighters were able to strand and kill the foreign invaders.
Just as it had thwarted Napoleon as well, the Russian winter played a key role in freezing the German invasion in its tracks. Mud initially slowed the German advance in autumn, then the bitter cold of winter set in. The Germans were not equipped for winter conditions, having set out in their summer uniforms. Despite the Wehrmacht’s mechanization, German forces still used horses extensively for the transportation of supplies, with many of the horses dying from the cold. Even machines could not stand up to the conditions; it got so cold that engines broke down and tanks and armored cars were rendered immobile. Thus, the German army, while still huge and powerful, was largely frozen in place in the winter of 1941 - 1942.
Incredibly, the Soviets were able to use this breathing room to literally dismantle their factories and transport them to the east, outside of the range of the German bombers. Whole factories, particularly in the Ukraine, were stripped of motors, turbines, and any other useful equipment that could be moved, and sent hundreds of miles away from the front lines. There, they were rebuilt and put back to work. By 1943, a year and a half after the initial invasion, the Soviets were producing more military hardware than were the Germans. Likewise, despite the relative success of the German invasion, Germany lost over 1.4 million men as casualties in the first year.