Before Barbarossa, the Germans and the Russians were allies, having signed an unexpected treaty shortly before the German and Soviet invasion of Poland. It was a non-aggression pact in which the Germans and the Russians had agreed how to divide up eastern Europe between themselves. The pact was unexpected because of their mutual hostility before that time. But Hitler had long wanted to destroy communism. So the pact was simply for short term convenience and the Nazis had no qualms about breaking it to pursue their ideological interests.
Barbarossa was largely the brainchild of Hitler himself. His general staff advised against fighting a war on two fronts. But Hitler considered himself a great Warlord, and indeed at this point in the war he had achieved a whole series of lightning victories against what appeared to be insurmountable odds. First, his brashness and willingness to take risks, combined with the discipline of his troops and the Blitzkrieg tactics, had won him the Sudetenland and Czechoslovakia with hardly a struggle, then Poland with only slightly more trouble. Then he achieved the rapid collapse of the French armies by running through Belgium and around the Maginot Line. Britain appeared to be holding out through sheer will. Hitler thought it was time to turn on his former friend in the East.
Hitler was overconfident after rapid success in western Europe, expecting victory in a few months and not preparing for a war lasting into the winter. He did not even equip his troops with cold weather gear. He hoped a quick victory against the Soviets would encourage Britain to accept peace terms.
Readers of Hitler's Mein Kampf (My Struggle) should not have been surprised to see him invade the Soviet Union. In that book, he makes clear his belief that the German people needed land, and that it was to be looked for in the East. Unfortunately, the book gained little attention under after Hitler was in power.
At the time it was the largest battle in history, between the two largest armies in the world.
The Germans were initially successful and almost reached Moscow. However, during the campaign, Hitler ordered the main thrust that had been heading toward Moscow to be diverted southward in order to help the southern army group capture the Ukraine. This move delayed the assault on the Soviet capital, although it also helped to secure the central army group's southern flank. By the time they turned their sites on Moscow, the autumn rains and eventually the winter snowfall ground their their advance to a halt. Thus they were prevented from much further gain, although this was also in part to stronger resistance from the Russians than had been expected. German logistics also became a major problem as a result of the great length of their supply lines.
The Nazi policy of killing, deporting or enslaving the "inferior" Russian population and using the land for "living space", did not help them gain support from within that population, despite any misgivings they may have had against their government under Stalin.
The turning point of the operation was when Nazi troops advanced within sight of the spires of The Kremlin. It was as close as they ever were to get, for Stalin's troops defended Moscow ferociously, and drove the Germans back into the frozen wastes of Russia as the winter advanced. The Germans decided to give up on Moscow for the time being, and focus on Stalingrad. But they were to have no more luck in that theater. After months of bitter hand to hand combat in the ruins of the city, the Germans were cut off from their supply lines and began to starve. In a brilliant military manouvre, the Soviets performed and encirclement operation and the Germans were trapped. Hitler ordered them to fight to the last man, and they displayed incredible fortitude and bravery under unbearable conditions. Many starved to death, and many others died of disease.
Near the end of the conflict, Hitler awarded General Paulus, head of the Stalingrad offensive, with the Field Marshall's baton. No German Field Marshall had ever been taken alive in war, and it is believed that Hitler awarded Paulus with the honor to ensure Paulus did not surrender. But after being the victim of blunder after blunder by Hitler and his General Staff, Paulus and his troops were demoralized, and the remaining holdouts finally surrendered to Soviet troops.
The victory by the Soviets at the Battle of Stalingrad was a major influence on the outcome of World War II. From that day on, the Soviets advanced from the East, while the United States and Britain advanced from the West.