After the June 1944 D-Day landings D'nitz withdrew his U-boat wolf-packs from the Atlantic convoy war and sent them into coastal waters, where they could harass the massive shipping movements necessary to supply the Allied armies advancing across Europe. Caught unawares by this change of strategy, the Allied anti-submarine forces were ill-prepared for the novel challenges of inshore warfare. It proved surprisingly difficult to locate U-boats that could lie silently on the seabed, and the shallow waters meant less than ideal conditions for sonar propagation. Furthermore, because the battle was nearer home, the U-boats wasted less time on transit, so at any one time there were more of them in combat. In the final months of the war there was also the threat of far more advanced and potent submarine types entering German service, but thanks largely to overwhelming numbers of escorts this last gamble by D'nitz was defeated. In fact, the Allied navies had never really established superiority, and this was to have enormous significance later during the Cold War, when the same tactics were planned by the Soviets. Since it had such a major impact on post-war naval thinking, it is a story of the utmost importance told by an accomplished U-boat author.