The most important thrust of the German invasion of Norway was the naval squadron assigned to steam up the Oslofjord, land troops, subdue Oslo and capture the King, Cabinet, and Members of Parliament. The battleship ‘Blucher’, carrying 2000 soldiers, (and a military band presumably to serenade citizens during the take-over), led the convoy followed by the pocket battleship ‘Lützow.’

Col. Birger Christian Erichsen, commander of Oscarsborg Fort that guarded the inner Oslofjord, had received several messages about the activities in outer Oslofjord during the night of April 8 – including the information from Pol III. Because of a misunderstanding, the German fleet passed the outer forts Bolærne and Rauøy and it was the radio station at Filtvet that advised Col. Erichsen that three large and several smaller warships had steamed by at low speed. Col. Erichsen had at his disposal two 55 year-old Krupp cannons, Moses and Aron, and some 24-year old torpedoes that had never been fired with live detonators. As ‘Blucher’ approached the fort, Erichsen gave the order to fire one shot from each cannon. It was 0421

Both shots hit ‘Blucher’s’ bridge and destroyed the communication control. In addition to troops, the ship carried ammunition and aviation fuel. The ammunition began to explode and several hits from smaller forts on the Drøbak side of the fjord ignited the fuel. As ‘Blucher’ slowed down and began to drift one of the two torpedoes from Oscarborg smashed into her and ignited an ammunition magazine. It was 0530. Thirty minutes later came the order to abandon ship. About 1000 men lost their lives. Another thousand managed to swim to shore. ‘Blucher ‘sank at 0622 The ‘Lützow’ sustained several hits from the shore batteries and had to limp away. The German attempt to surprise the Norwegian forces had failed.