The successful 6 June 1944 D-Day landings were only the opening scenes in a naval drama that continued to unfold on the beaches of Normandy. As Operation Neptune's planners had recognized, once U.S., British, and Canadian troops began getting ashore, the Allies were in a frantic race with the Wehrmacht to build up strength at the point of contact.
For the Western powers this came down to moving huge numbers of men and amounts of materiel across the English Channel while protecting the convoys from German air and naval attacks. At the same time, they needed to interdict the enemy's movement of vehicles, equipment, and personnel to the invasion area from elsewhere in Europe. The winner of this contest would be in the best position to decimate its opponent's forces, and either (in the German case) push the invaders back into the Channel or (in the Allies' case) punch a hole through the enemy's defenses and break into the open French countryside.