In the spring of 1940, the German army began its offensive against northern and western Europe. In less than three months, it would conquer the entire European littoral from North Cape in Norway to the Spanish frontier. After the entry into the war of the United States in December 1941, the German high command sought to prevent at all costs the opening of a second front in the west. It decided to construct a wall of concrete and steel along more than 600 kilometres of coastline to render all landings impossible.