"Germany’s invasion of Russia with more than three million men was the largest surprise attack in military history."
The Nazis and the Communists had been allies for almost two years. Joseph Stalin ignored repeated warnings about a possible German attack.
"German troops killed or wounded 150,000 Soviets in the first week of the campaign, while the Luftwaffe—the Nazi air force—destroyed over 2,000 Russian planes in just the first two days... Most outside analysts began predicting that a Russian defeat was only weeks or even days away. Despite these early setbacks, the Soviets’ seemingly inexhaustible supply of troops ultimately proved too much for the Germans to overcome...
"In addition to the might of the Red Army, German troops were also worn down by 'General Winter'—the nickname used to describe the deadly Russian frost."
Victor Davis Hanson says that the best tank design of WWII, by far, was the Russian T-34: "The Germans invaded Russia without knowing that the Russians had T-34 tanks—proof of how threadbare their pre-invasion intelligence was."
UPDATE: More here on what made that tank so good.
A short video of a recent demonstration at a war show.