One of the classic “what ifs” of the Second World War centers on how—or if—the German invasion of the Soviet Union in June 1941, code-named Operation Barbarossa, could have achieved a quick victory. Hitler certainly believed that it could. All one had to do, he insisted, was to “kick in the door” and the “whole rotten structure” of Stalin's Communist regime would come tumbling down. In many respects Barbarossa was a stunning success. The Germans took the Soviets completely by surprise, advanced hundreds of miles in just a few weeks, killed or captured several million Soviet troops, and seized an area containing 40 percent of the USSR's population, as well as most of its coal, iron ore, aluminum, and armaments industry. But Barbarossa failed to take its capstone objective, Moscow. What went wrong?