 "Fliegerführer Atlantik" responded by providing fighter cover for U-boats moving into and returning from the Atlantic and for returning blockade runners.Nevertheless, with intelligence coming from resistance personnel in the ports themselves, the last few miles to and from port proved hazardous to U-boats.Dönitz's aim in this tonnage war was to sink Allied ships faster than they could be replaced; as losses fell and production rose, particularly in the United States, this became impossible.