ON JUNE 22, 1941, at 3:15 in the morning, the largest invasion force ever assembled crossed into the Soviet Union from the west to begin Operation Barbarossa. Three million German troops, another half a million soldiers from allied countries such as Hungary and Romania, 3,600 tanks, 600,000 motor vehicles, 7,000 artillery pieces, and 2,700 combat aircraft opened the assault. As bombs rained down on Soviet cities and airfields, the Germans advanced up to 50 kilometers a day, taking the opposing Red Army forces by surprise, killing and capturing huge numbers of Soviet soldiers. Confused and disoriented, the Red Army virtually collapsed. Retreat was made difficult by the Germans’ destruction of roads, railways, and communications behind the Soviet front. By July 3,...