How has to fight on the eastern front, had a few months before the German invasion, Stalin understood that Hitler was preparing to attack the Soviet Union and well prepared to defend (ordered the mobilization of several million additional soldiers to the Red Army, construction field fortifications in Belarus and Ukraine, increasing the production of weapons to the level of 1942, released from prison all the officers imprisoned during the Great Purge etc.)? Is Red Army could stop the German offensive on the line of the Dnieper and the Daugava river, instead of Moscow, Leningrad and Rostov? How this would affect the further course of the war?