Colonel Birger Eriksen was commander of Oscarsborg Fortress. In the early hours of April 9, 1940, German ships entered the fjord to take Oslo. At the head of the fleet association was the heavy cruiser "Blücher", at that time the most modern ship in the German Navy. The test drives had only been completed on March 30, 1940, it was the first use of the "Blücher". The heavy cruiser "Blücher" was hit several times from Oscarsborg Fortress. The ship caught fire and drifted across the narrow stretch of fjord and closer to the old fortress. Directly in range of the torpedo batteries. Two 40-year-old torpedoes then hit the "Blücher" and she finally sank in the fjord without firing a single shot and only a few days after the last sea trial. The fort's guns were then 47 years old and the torpedo batteries similarly antiquated. The successful resistance of the fortress under Colonel Birger Eriksen made it possible for the Norwegian King Håkon VII to escape to Great Britain into exile with the country's gold treasure.