Why was the USSR unprepared for German invasion of 1941?

Stalin had recieved numerous warnings that an attack must be imminent from many sources. He, however, rejected all such reports, refusing to believe that Hitler would risk fighting a war on 2 fronts. Soviet forces weren't allowed to respond in any way that the Germans could interpret as confrontational. These early days exposed Stalin's greatest error: misjudgement of Hitler's actions and intentions. This, together with his ill-judgement and ill-timed purge of the armed forces, amounted to a colossal failure of leadership.

Germany had an experienced and well-trained force which was mobile and well-prepared with good communications and a ruthless approach…