The Battle of Drøbak Sound took place in the northernmost part of the Oslofjord on 9 April 1940, on the first day of the German invasion of Norway. It was the start of the war in Western Europe—and an end to the "Phoney War". In the end, the fortress' armament worked flawlessly. By sinking the lead ship of the German armada headed for Oslo, Oscarsborg Fortress saved the Norwegian king and government from being taken captive in the first hours of the invasion.