Allied air forces developed tactics and technology to make the Bay of Biscay, the main route for France-based U-boats, very dangerous to submarines. The Leigh Light enabled attacks on U-boats recharging their batteries on the surface at night. "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.