During the Second World War the submarine became the principal German weapon at sea. This time they took the fight to the Atlantic Ocean because the Atlantic coasts from France up to Norway had fallen into German hands. The anti-submarine tactics used by the Allies at the end of the First World War appeared to have lost their usefulness in the ocean. The wolfpack tactic (hunting in group) enabled the German submarines to attack convoys as well. Only when the Allies gained air supremacy and the flight range of the Allied bombers sufficed to cross the entire Ocean did the tide turn.
The Allies gained air supremacy already at the start of the war, in particular after the Battle of Britain in 1940. Hitler gave up his invasion plans for Great Britain after this battle. From that moment up to the landings in Normandy, the coasts from the Bay of Biscay up to the Barents Sea constituted the Western Front. The initially aggressive German attitude became defensive. Especially when Operation Barbarossa (the invasion of Russia in 1941) failed and the Eastern Front required an increasing number of troops, the German High Command tried to tackle the shortage in soldiers by further developing the coastal defences under the name Neue Westwall. In August 1942 the coastal defence line was renamed the Atlantikwall and reinforced with various construction programmes that became more intense after the Dieppe Raid.
The failure of the Dieppe Raid resulted in the Allied forces developing a new invasion plan based on experiences, information from resistance fighters, aerial photographs, postal cards, field exercises etc. The invasion took place almost 2 years later on 6 June 1944. The Atlantikwall did not hold out long, and the liberation of Western Europe had begun.