Anchor Lisa Mullins speaks with Andrew Nagorski, author of "The Greatest Battle: Stalin, Hitler, and the Desperate Struggle for Moscow That Changed the Course of World War Two." The book describes the German invasion of the Soviet Union in 1941, and Stalin's response. The Soviet dictator was at times indecisive, but in the end clamped down hard on his own people in order to tighten his hold on the city.