The shortest answer is resources. Germany went to war with the world's largest economies and armies all at once. The British had the largest empire with the largest navy and a fairly strong army. The french had a large capable army, airforce and navy. The Soviets had the largest land army in the world. The US had the largest industrial base and the second largest navy in the world. The amount of men, material, food, ships, war machines, and fuel the allies had was simply overwhelming for any one nation to handle. The Germans could not bomb the furthest soviet factories nor the american factories, after the battle of britain the Royal airforce always maintained air superiority, all while German factories and cities were bombed daily. The German surface navy was crippled during the campaign for norway and would never again be more than a fleet in being and raiding force. The joint British, Canadian, and American effort had won the battle of the atlantic by 1943 with full air cover and escort warships for the merchant ships. With no german surface fleet capable of defending the coast against anything more than a raiding force, a crippled airforce by both allied actions and logistical decision, the army having been exhausted with its supply lines constantly bombed. Germany had no chance of winning.