 Eventually the Command was given the investment it needed. Radar and long-range aircraft enabled the Command to hunt down and destroy U-boats with growing efficiency. German submarines had been sinking a large number of Allied ships in the Atlantic Gap, which was a stretch of water in the central Atlantic beyond the range of most Allied aircraft. The covering of the gap by long-range aircraft equipped with radar helped reduce the effectiveness of U-boats. In May 1943 the campaign reached a peak, when a large number of U-boats were sunk with little loss to Allied shipping. During this time Coastal Command gained the initiative. It was known by the Germans as Black May. Thereafter the suppression of German submarines was effective not only in the Atlantic, but also in their transit routes through the Bay of Biscay in 1942, 1943 and 1944. In June 1944 the Normandy landings and subsequent Operation OVERLORD liberated France and cost the Germans their air and submarine bases, won in 1940. The U-boats were thus forced to relocate to Norway and Germany in August, restoring many of the difficulties faced by the Kriegsmarine in 1939 and early 1940. The entire strategic position, which had been the foundation of the U-boat war since June 1940 had been undermined.
