By the end of 1943 the German submarine war on Atlantic convoys was all but defeated, beaten by superior technology, code-breaking and air power. With losses mounting, Doenitz withdrew the wolfpacks, but in a surprise change of strategy, following the D-Day landings in June 1944, he sent his U-boats into coastal waters, closer to home, where they could harass the crucial Allied supply lines to the new European bridgehead. Caught unawares, the British and American navies struggled to cope with a novel predicament -in shallow waters submarines could lie undetectable on the bottom, and given operational freedom, they rarely needed to make signals, so neutralizing the Allied advantages of decryption and radio direction-finding. Behind this unpleasant shock lay an even greater threat, of radically new sub- marine types known to be nearing service. Doenitz saw these as war-winning weapons, and gambled that his inshore campaign would hold up the Allied advance long enough to allow these faster and quieter boats to be deployed in large numbers. This offensive was perhaps Germany's last chance to turn the tide, yet, surprisingly, such an important story has never been told in detail before. That it did not succeed masks its full significance: the threat of quiet submarines, operating singly in shallow water, was never really mastered, and in the Cold War that followed the massive Soviet submarine fleet, built on captured German technology and tactical experience, became a very real menace to Western sea power. In this way, Doenitz's last gamble set the course of post-war anti submarine development.