German underestimation of the Soviets
When the German armies crossed the Russian frontiers and initiated Operation Barbarossa in 1941, they entered enemy territory unequipped for Russian weather, supremely confident, at least at the level of Hitler, the Soviets would capitulate before autumn. The early successes achieved by the Germans served to reinforce this overconfidence. But it wasn’t long before the Germans discovered that they weren’t going to shoot the Soviet Air Force out of the sky, and as they drove deeper into Russia the enemy resistance stiffened. Logistics became a problem as the supply lines grew longer. Pauses by the German advances to regroup were met with counterattacks.
By autumn the Germans were forced to a halt with advance units of the German army within sight of Moscow. It was as far as they would go. New Soviet weaponry, including tanks superior to those of the Panzer divisions shifted the initiative to the Russians. A seemingly inexhaustible supply of fresh troops swelled the Russian lines. An old Russian ally, the weather, shifted into their favor and the Germans were ill-equipped to fight in the increasingly inclement conditions. Lend Lease supplies from the United States bolstered the Russian logistical situation. An increase in partisan activity behind the front lines meant more and more garrison troops were needed to protect supply lines. The Soviets were far better prepared than the Germans estimated, and the Germans paid for the mistake.