On June 22, 1941, Nazi Germany and its Axis allies began a massive invasion of the Soviet Union named Operation Barbarossa, in which nearly 4.5 million troops launched a surprise attack to realize Hitler's ambition of a vast eastern imperium. 

Although Germany had signed a nonaggression pact with the Soviet Union in 1939, both sides remained suspicious of one another, and the agreement gave them more time to prepare for a probable war.

Even so, the Soviets were unprepared for the sudden Blitzkrieg attacks across a border that spanned nearly 1,800 miles, and they suffered horrible losses. Within a single week, German forces advanced 200 miles into Soviet territory, destroyed nearly 4,000 aircraft, and killed, captured, or wounded some 600,000 Red Army troops. By December 1941, German troops were within sight of Moscow and had laid siege to the city when their offensive ground to a halt. 

By the end of Barbarossa, the largest, deadliest military operation in history, Germany had suffered close to 775,000 casualties. More than 800,000 Soviets had been killed, and an additional 6 million Soviet soldiers had been wounded or captured.

Marcus Jones, history professor at the U.S. Naval Academy and consultant for the Institute for Defense Analyses, explores how Hitler's plan to conquer the Soviet Union before winter failed at great cost—and why that failure would prove to be a turning point in the war.