Operation Barbarossa
When Hitler broke his non-aggression pact with Stalin and invaded Soviet territory in 1941, he did so in a massive way, intending to secure a quick victory. He sent over 3 million soldiers, and 3,000 tanks were involved in the attack. The German generals had predicted that they could win in 10 weeks' time. As we know, that didn't happen, and the war on the Eastern front between Germany and the USSR became the bloodiest theater of war in World War II.
