[14] The fortress' armaments worked flawlessly despite their age, sinking the Blücher in the sound and forcing the German fleet to fall back.

He was well aware that Norway was officially neutral, but that the government was inclined to side with the British in case of direct Norwegian involvement in the war.

As an unidentified flotilla started forcing its way past the outer fortifications in the south of the Oslofjord, late at night on 8 April, Eriksen called Anderssen in and had him come down to the fortress.

[16] When Anderssen had been called back into duty a month previously, he had been a pensioner for 13 years, having originally retired from his post as commander of the torpedo battery in 1927.

[18] At 04:21 on 9 April, Eriksen gave the Main Battery guns the order to fire at the lead ship of the unknown flotilla forcing its way towards Oslo.

[24] In all, thirteen 15 cm rounds and around thirty 57 mm shells hit the German cruiser as it passed the guns of the fortress' secondary batteries.

Blücher's fire-fighting system was also knocked out by shell fragments from the two Norwegian batteries, making attempts to control the fires aboard the ship and rescue the many wounded much more difficult.

[32] After passing the line of fire of the fortress' gun batteries, the cruiser was burning and severely damaged, but her captain still hoped he would be able to save his ship.

At this point, however, Blücher entered the sights of Kommandørkaptein Andreas Anderssen as she slid past the torpedo battery at a range of only 500 m (550 yd).

As Anderssen had overestimated the speed of his target slightly, the first torpedo hit near Blücher's forward turret (nicknamed "Anton"), creating only inconsequential damage.

[23] This caused catastrophic damage to the cruiser and blew open many of her bulkheads, allowing water to flood her decks while she was burning furiously.

[34] The crew's struggle ended when, at 05:30,[34] fires reached a midship ammunition hold for the 10.5 cm (4.13 in) Flak guns, blowing a large gap in the ship's side.

[35] At 06:22, Blücher sank bow first into the depths of the Oslofjord, first lying over on her port side, then turning upside-down and finally succumbing with her screws the last to disappear below the surface.

[9] After the ship had disappeared from the surface, large quantities of oil floated up and covered the close to two thousand sailors and soldiers fighting for their lives in the freezing water.

[7] Obergefreiter Günther Morgalla—who survived the sinking—later said that, swimming toward his shivering crew mates onshore, he heard someone defiantly singing the Deutschlandlied followed by "Das kann doch einen Seemann nicht erschüttern."

[38] Around 1,000 of the Germans, including Generalmajor Erwin Engelbrecht and Admiral Oskar Kummetz, were eventually moved to a nearby farm and placed under light guard.

[42] By the time Blücher sank, the remaining naval force destined for Oslo had long since turned around and retreated back down the fjord.

During the battle, another burning ship was spotted in the distance from Oscarsborg, leading the Norwegian defenders to believe they had sunk another German warship in addition to Blücher.

[1][11][12] Brummer was indeed lost in connection with the invasion, but only when she was on her way back to Germany on 14 April, when she was torpedoed by the Royal Navy submarine HMS Sterlet and sank the next day.

After a break in the attacks from 12:00 to 13:30, during which time Lützow bombarded Hovedøya,[1] the Luftwaffe bombers returned at 13:30 and soon strafed the remaining Norwegian anti-aircraft guns, forcing the crew to seek shelter in the nearby forest at around 14:00.

[47] Although the German naval attack on Oslo had been thwarted by the actions of Oscarsborg, the city was seized later that day by forces that were airlifted into Fornebu Airport.

In light of the fall of the capital, and with news of German landings at the village of Son south of Drøbak,[46] Colonel Eriksen decided that further fighting without adequate infantry support was in vain, and agreed to a ceasefire in the evening of 9 April.

[48] In one of the more peculiar battles of the war, a hundred year old fortification, manned by raw recruits and pensioners and armed with 40- to 50-year-old weaponry of German and Austro-Hungarian manufacture, had destroyed a ship so new that its crew was still finishing training.