Hitler DID attack in the summer; Barbarossatag was June 22nd 1941, Napoleon invaded June 24th, 1812. Napoleon took Moscow, and held it through the winter until he abandoned the city, and began his disastrous retreat into Poland. Hitler's armies smashed the Red Army, encircling huge numbers of Russian troops at Smolensk, Minsk, Bialystock, Uman and Kiev. However, the Germans also suffered huge losses, and had not planned for a long campaign. It was expected that the Ostheer would have destroyed the Red Army by September at least, hence why the Germans only had enough winter clothing for 1/3 of their troops.

Max Hastings, "All Hell Let Loose"; Robert Kirchubel, "Operation Barbarossa"