The German task group with shortest distance to travel was led by the heavy cruiser Blucher up the Oslofjord to the Norwegian capital Oslo. The Oscarborg fortress lay on an island in the middle of the Fjord and proved to be a deadly obstacle. The Commander of the Fortress was uncertain whether the ships he faced were English or German. Germany was relying on surprise and subterfuge to gain an early advantage and had not declared war. Norway was still neutral so it was not clear whether the shore battery could open fire within the standing rules of engagement.
The 64 year old Colonel Eriksen decided that he could open fire, because the ships had passed another fortress further down the Fjord which had fired warning shots, and at 0421hr declared:
“Either I will be decorated, or I will be court-martialed. Fire!”
From a range of approximately 1800 metres the two 11inch (28cm) shells from the Krupps guns that were capable of firing caused massive damage to the Blucher. The first round entered a magazine causing explosions and setting the Blucher on fire, the second shell disabled the electrical supply used by the guns. The fortress was largely manned by reservists and recruits who had been called up within the last week, and they were not yet trained up to reload during the time it took for the Blucher to pass them.
The fatal damage was done a few minutes later by a shore based torpedo battery commanded by Commander Andreas. He had recently replaced the regular commander who had fallen ill. Anderssen had been retired for thirteen years and had only been called in to take over because he lived locally. The antiquated Austro-Hungarian torpedoes were of unknown reliability but they both detonated, sinking the Blucher. From the crew of 800 and the 1500 troops on board the Blucher, intended for the occupation of Oslo, only around 200 survived.
The remaining ships in this task group were forced to land their troops some 50 miles away from their objective, Oslo. The interruption to the German plans gave the Norwegian Royal Family the opportunity to escape. The capture of Oslo itself was only delayed by a matter of hours, it was taken by airborne troops later that day.
Bismarck-Class has images of the Oscarborg Fortress as it is today.