Oscarsborg — the intruder in the Oslofjord

Colonel Birger Eriksen, 64, has commanded since 1933 the Oscarsborg fortress, an old work installed on islets at the narrowest point of the Oslofjord, some thirty kilometres south of the Norwegian capital. A career artillery officer, he has a reduced garrison largely composed of recruits. His principal pieces are three Krupp 280 mm guns dating from the late nineteenth century, nicknamed Moses, Aaron and Joshua, and a submerged torpedo battery on the opposite bank.

During the night of 8-9 April 1940, Norway is officially neutral. The country has not mobilised, and no clear order comes down from Oslo. Around one in the morning, unidentified vessels, all lights masked, are coming up the dark channel toward the Oscarsborg narrows. Peacetime instructions provide for warning shots before any live fire; some instructions even require an intruder to be summoned to identify itself.

Eriksen does not know that this is the German squadron led by the heavy cruiser carrying the staff charged with taking the capital. The vessels are coming within range. He must decide in a few minutes, without superior orders.