The German generals all thought Russia was weak due to Stalin's purges, and they all thought war was inevitable so they all thought it better to get it over with before Russia could become very strong. British forces were hurled into the sea at Dunkirk, so there was no two front war. If the Germans were so clever, they would have annihilated the British at Dunkirk instead of letting them escape. Was that Hitler's fault, or the responsibility of the Generals on the ground?
