Excerpts from Books and Wikipedia
"German failure to capture Moscow doomed the Nazi attempt to conquer the Soviet Union." [Davis: 100 Decisive Battles, p. 386] "In August 1941, German forces captured Smolensk, an important stronghold on the road to Moscow. At this stage, although Moscow was vulnerable, an offensive against the city would have exposed the German flanks. In part to address these risks, in part to attempt to secure Ukraine's food and mineral resources, Hitler ordered the attack to turn north and south and eliminate Soviet forces at Leningrad and Kiev. This delayed the German advance on Moscow. When that advance resumed on 2 October 1941, German forces had been weakened, while the Soviets had raised new forces for the defence of the city." [Wikipedia] "This diversion of troops immediately after the Nazi victory at Smolensk was viewed by the Soviet high command as nothing short of a miracle." [Davis: 100 Decisive Battles, p. 387]