Sep. 30, 1941 to November 30, 1941 would be the time, although October 16, 1941 would be bull eye hit. Nazi broke through Soviet frontline Sep. 30, 1941 and situation in Moscow October 16-17, 1941 is best described by two words - anarchy and pandemonium. Stalin, for all his sins, could be credited with organizing Soviet leadership to overcome it. With him dying Oct. 17, Soviets will likely lose a week or so organizing new executive hierarchy, and by the time they're done it could be too late. Nazi occupation of Moscow will not lead to immediate disintegration of the Soviet Union, but will be bloody painful, with Moscow being not only "spiritual" center of the country, so to speak, but by far biggest transport intersection and industrial area as well.