During WW2, most people believed Germany would quickly crush the Soviet Union in 3 to 6 months. It took three and a half years and they lost.
Operation Barbarossa was intended to deal a total defeat to the Soviets in only three to six months, but in the early days of the invasion, many thought the fall might come even sooner. 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.
As German tanks and troops swarmed through Soviet territory in a three-pronged attack, 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.
While the invaders succeeded in knocking several million Russian soldiers out of the war by November 1941, they had also suffered more than 700,000 casualties of their own. Following a series of ferocious counterattacks by the Soviets, the Nazis were forced to abandon all hope of a swift victory. The war would drag on for another three and a half years.