By autumn 1940 the first round of fighting in World War II was over. In northern Europe, German forces occupied Poland, Norway and Denmark. Both sides recognized that further operations demanded naval and air superiority in northern waters. Germany needed free access to the Atlantic Ocean through the North Sea; Britain had to prevent that access, which threatened the lifeline to the United States. More than ever before, it became essential for both sides to have meteorological information from the northern Atlantic Ocean area. Germany's need was especially acute, for the routes for her shipping from ports in Scandinavia traversed enemy-patrolled waters, where foul weather was essential for evasion.