The purpose of the Atlantic Wall was to prevent enemy landings along the coast stretching from Norway to the Pyrenees and its development can be explained by Nazi Germany changing military position. In June 1940, following German victories over the Netherlands, Belgium, and France, four heavy navy batteries were set up in the Calais-Boulogne sector. The purpose of these batteries was not to defend the coast but rather to back up Operation Sealion, the planned invasion of England. The Walls Foundation. When the United States joined the War in December 1941, the German high Command was forced to change its defence strategy with regard to the western coasts of the Nazi empire. In a directive from the German Supreme Command dated December 14th 1941 it was stated that the German forces should be able successfully and surely to resist any landing attempts even by the largest enemy forces, using the fewest possible permanently stationed troops.The directive went on to state that the aim was to construct a series of coastal batteries to ensure long range coastal defence. Hitler considered this as a matter of some urgency. It is interesting to note that the coast of Normandy came third on the German list of possible sectors for a seaborne attack, after Norway and Northern France. The War on the Eastern front dragged on and the German High Command was forced to send in fresh troops from the West, thus weakening its defences on this front. Seeing the danger, Hitler drew up a plan for the defence of the coasts of Europe. These instructions formed the famous #40 directive of March 23rd 1942 known as the Kustenverteidigung or Coastal defence directive. This directive indicates the extent of the German High Command concern over the Allied landing. The fact that the fortifications had to be extremely resistant is stressed the fortified sectors and strongpoints must be designed so as to be able to resist prolonged enemy attacks, even if the enemy forces are larger. They must be defended to the end. The initial objective was, however to destroy the enemy before he is able to land, as far away from the coast as possible, or if necessary, as soon as he sets foot on the beach.