As D-Day approaches, Allied control of the Atlantic is vital. In May 1943, a massive U-boat force targets a convoy, triggering a brutal multi-day battle with the Royal Navy that helps decide the outcome of the Atlantic war.As D-Day approaches, Allied control of the Atlantic is vital. In May 1943, a massive U-boat force targets a convoy, triggering a brutal multi-day battle with the Royal Navy that helps decide the outcome of the Atlantic war.