The 'Battle of Drøbak Sound' was fought between German naval and Norwegian coast-defence forces in Drøbak Sound, the northernmost part of the outer Oslofjord in southern Norway, as the German 'Weserübung' invasion of Norway began (9 April 1940).

A German naval force commanded by Konteradmiral Oskar Kummetz, Gruppe V (heavy cruiser Blücher, heavy cruiser Lützow, light cruiser Emden, three torpedo boats and eight minesweepers) transported 2,000 men of Generalmajor Erwin Engelbrecht’s 163^rd Division to Oslo, while four minesweepers moved 150 troops to Egersund. Late in the evening of 8 April, Gruppe V was spotted by the Norwegian guard vessel Pol III, which was destroyed. Led by Blücher, Gruppe V had been despatched up the Oslofjord to begin the German invasion of Norway with the primary objective of seizing Oslo, the Norwegian capital, and capturing King Haakon VII and his government. The force was engaged in the fjord by the Oscarsborg fortress, an ageing coastal defence installation near Drøbak, that had been relegated to the task of training coastal artillery gunners, a fact which had led the Germans to disregard its defensive value. However, unknown to German military intelligence, the fortress’s most powerful weapon was a torpedo battery, which would be used to great effect against the German invaders.

The fortress’s armaments worked flawlessly despite their age, sinking Blücher in the sound and forcing the German force to fall back. The loss of the German flagship, which carried most of the troops and Gestapo agents intended for the occupation of Oslo, delayed the German occupation long enough for the king and his government to escape from the capital.

As the political situation between Germany and Norway was chaotic in April 1940, the 64-year-old Norwegian commander, Oberst Birger Eriksen, had received neither clear orders nor notice as to whether the warships his command’s area of responsibility were German or Allied. Eriksen was fully aware of the fact that Norway was officially neutral, but that the government was inclined to side with the UK in case of direct Norwegian involvement in the war.

Apart from the officers and non-commissioned officers, almost all the men garrisoning the fortress were fresh recruits conscripted only seven days earlier on 2 April. Because of the influx of 450 recruits, the fortress’s naval mines had not been deployed by 9 April. Part of the recruits' training was to lay the mine barrier, a process planned for implementation a few days later.

The commander of the torpedo battery at Oscarsborg had at the time of the battle been on sick leave since March 1940, and as a result the retired Kommandørkaptein Andreas Anderssen, who lived in nearby Drøbak, had been assigned as temporary commander. 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 summoned Anderssen to the fortress. Anderssen was transported by boat over the fjord to the torpedo battery: having first served at the torpedo battery in 1909, Anderssen knew the ageing weapons intimately. On being recalled to service one month previously, Anderssen had been a pensioner for 13 years. The battery had three torpedo tunnels which could fire six torpedoes without reloading, and nine torpedoes were stored and ready for use.

While the main combat position for the main battery and the fortress commander were located on the island of Håøya to the north-west of Søndre Kaholmen, as a result of the special circumstances in 1940, Eriksen commanded from the reserve station on the eastern flank of the main battery at Søndre Kaholmen.

At 04.21 on 9 April, Eriksen gave the guns of the main battery the order to fire at the leading ship of the unknown flotilla forcing its way toward Oslo. Upon giving the command, Eriksen was questioned, and responded with 'Either I will be decorated or I will be court martialled. Fire!' Two rounds from the 280-mm (11.02-in) Krupp guns Moses and Aron engaged the German heavy cruiser Blücher at a range of just 1,970 yards (2000 m). The two Norwegian guns had been loaded with 562-lb (255-kg) high-explosive shells, and the firing of these guns 'in anger' constituted a violation of the pre-war Norwegian rules of engagement, which dictated the initial firing of warning shots, as had been the case at the Rauøy and the Bolærne fortresses farther down the fjord. Eriksen later explained his decision by alluding to the fact that the German naval force already had forced their way past the fortresses farther to the south in the Oslofjord and had received both warning shots and live rounds from these more outlying coastal fortifications. As the vessels had continued up the fjord toward the capital, Eriksen was of the opinion that he had every right to consider them enemy warships and to engage them as such.

The first 280-mm (11.02-in) shell hit Blücher just forward of her after mast, and set the midships area up to the foremast on fire. The second round struck the base of one of the forward 203-mm (8-in) gun turrets shortly thereafter, throwing large parts of it into the fjord and igniting further fires. There was time for the main battery to fire only these two rounds as a result of the long time it took to reload with only 30 untrained recruits. Only one crew of trained gunners was available, and two guns had been made operational only by splitting the trained gunners between the two guns and using non-combatant privates to assist. Thus there was no time to reload, or even to fire the third gun, Josva, which was loaded but unmanned.

The reason for the significant effect of the two 280-mm (11.02-in) rounds on Blücher was that the first round penetrated the side of the ship and exploded inside a magazine containing barrels of oil, smoke dispensers, incendiary bombs, aircraft bombs for the cruiser’s Arado Ar 196 single-engined reconnaissance floatplanes, and depth charges. The bulkheads on that deck were blown out and the burning oil developed into an intense fire. The second 280-mm (11.02-in) shell also knocked out the electrical circuitry for the ship’s main guns, rendering them unable to return fire.

While fire raged aboard Blücher, the Norwegian secondary coastal batteries fired at the cruiser with guns ranging in calibre from the two small 57-mm (2.24-in) pieces at Husvik, intended to protect the fortress’s missing mine barrier, to the three 150-mm (5.91-in) guns of the Kopås battery on the eastern side of the fjord. The larger guns wrought havoc on Blücher, while the smaller guns concentrated on the cruiser’s superstructure and her 37- and 20-mm anti-aircraft weapons, and were partially successful in suppressing the fire from the cruiser’s small-calibre guns as Blücher slowly steamed past the fortress. The Husvik battery had to be abandoned when Blücher passed in front of it and fired her light anti-aircraft guns directly down into the positions. Although the main building at the battery caught fire, the Norwegians suffered no casualties. In all, 13 150-mm (5.91-in) rounds and around thirty 57-mm rounds hit the German cruiser as she passed the guns of the fortress' secondary batteries. One of the 150-mm (5.91-in) rounds from Kopås disabled Blücher's steering gear and forced the cruiser’s crew to steer using the engines to avoid running aground. Blücher's firefighting system was also knocked out by shell fragments, making very difficult all efforts to control the fires aboard the ship and rescue the many wounded.

As the now crippled Blücher passed the fortress’s guns, a sudden outburst of voices from the burning ship could be heard above the noise of battle: Norwegian sources state that the crew started to sing Deutschland, Deutschland über alles. Only at this point did it become clear to the men of the fortress whom they were fighting. Later, at 04.35, Eriksen received a message from the Norwegian minesweeper Otra confirming that the intruding ships were German. The message had been sent to the naval base in Horten at 04.10, but the massive communications problems that severely hampered the efforts of the Norwegian military throughout the Norwegian campaign had prevented it from reaching Oscarsborg before the start of the battle.

Blücher's return fire was ineffective, with the smaller-calibre weapons mostly pointing too high and the main battery’s 203-in (8-in) guns unable to fire as a result of the damage caused by the second 280-mm (11.02-in) round from Oscarsborg’s main battery. The shelling lasted only five to seven minutes. When the guns on each side fell silent, and with all the 800 embarked men of the 186^rd Division still below deck, 'there was a dead silence on board the whole ship, no movement whatsoever was identified'.

After passing the arcs of the fortress’s gun batteries, the German 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 Anderssen’s weapons as she slid past the torpedo battery at a range of only 545 yards (500 m). The torpedoes were 40-year-old Whitehead weapons of Austro-Hungarian manufacture, and had been practice-launched considerably more than 200 times before, but no one was certain whether or not they would function. As Anderssen pushed the torpedoes' firing mechanism button at about 04.30, the weapons functioned perfectly: first one and then the other raced out of their underwater tunnels 9.8 ft (3 m) below the surface of the water toward the burning warship. As Anderssen had overestimated the speed of his target slightly, the first torpedo hit near Blücher's foremost turret, causing only inconsequential damage. The aim was corrected for the second torpedo launch and the torpedo struck Blücher amidships, hitting the same general area as the first 280-mm (11.02-in) shell. This caused catastrophic damage to the cruiser and blew open many of her bulkheads, allowing water to flood her decks as the ship burned furiously. The third torpedo launcher was left loaded in case more ships were to follow close behind Blücher. After firing, the other two tubes were reloaded and readied for the next target.

With her engines knocked out by the second torpedo hit, the cruiser anchored near the Askholmene islets just to the north and out of the arc of fire of the fort’s guns in order to fight the fires raging throughout the vessel. Blücher's torpedoes were fired against the land to prevent them from exploding in the uncontrolled fires. The crew’s struggle ended when, at 05.30, fire reached a midship ammunition magazine for the 105-mm (4.13-in) anti-aircraft guns, blowing a large hole in the ship’s side. The blast also ruptured the bulkheads between the boiler rooms and tore open the cruiser’s fuel bunkers, igniting more fires. By this point, Blücher was doomed and, at 06.22, sank bow first into the depths of the Oslofjord, first listing over to port then turning upside-down and finally succumbing with her propellers the last elements to sink below the surface. After the ship had disappeared, large quantities of oil floated to the surface and covered the close to 2,000 sailors and soldiers fighting for their lives in the freezing water. The oil rapidly caught fire, killing hundreds more Germans.

In all, between 650 and 800 Germans died, and 550 of the approximately 1,400 wet and cold survivors were taken prisoner by men from the No. 4 Company of the Norwegian Royal Guards under the command of Kaptein A. J. T. Petersson. In total, about 1,200 of the survivors had made it ashore at Frogn near Drøbak. The guardsmen were supposed to take all the Germans prisoner, but mainly focused on caring for the many wounded and dying. Around 1,000 of the Germans, including Engelbrecht and Kummetz, were eventually moved to a nearby farm and placed under light guard. None of the prisoners was interrogated. By 18.30, the Norwegian soldiers withdrew from the area, abandoning the Germans, and Engelbrecht and Kummetz then made their way to Oslo, which they reached at 22.00 and then moved into the Hotel Continental, though without most of the troops intended to occupy the capital. Many of the German wounded were initially taken to the summer hotel in Åsgårdstrand for medical care, where Norwegian wounded had already been brought. The hotel was the temporary and wholly improvised location for the Royal Norwegian naval hospital, which had been evacuated from Horten at 00.00 on 8 April.

By the time Blücher sank, the remainder of Gruppe V destined for Oslo had long since turned back and retreated down the fjord. Seeing the geysers of water from underwater explosions on Blücher, and unaware of the torpedo battery, Kapitän August Thiele, commander of the heavy cruiser Lützow (the recently renamed 'pocket battleship' Deutschland), came to the conclusion that the flagship had hit one or more mines, and at 04.40 decided that Gruppe V should turn back and land the invasion forces out of range of the Oscarsborg batteries. The planned coup against Oslo to force the surrender of the Norwegian government was replaced by a land advance up the Oslofjord.

As the group made good its escape, the fortress managed to damage Lützow, the 150-mm (5.91-in) guns of the Kopås battery scoring three hits and knocking out the ship’s forward 280-mm (11.02-in) turret. Kopås kept firing at the retreating ships until they disappeared in the mist at a range of around 3,280 yards (3000 m). After pulling out of range of the fortress guns, Lützow employed her remaining triple turret to bombard the defenders from a range of 9,840 to 10,935 yards (9000 to 10000 m) 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. For some time after the battle, the belief was that Oscarsborg had sunk the artillery training ship Brummer, but the reality was that the burning ship was the 107-ton Norwegian cargo cutter Sørland, which had stumbled into the battle while on her way from Moss to Oslo with a cargo of paper. As the little Norwegian vessel had mistaken the events as a military exercise, she maintained her course until she came under fired and was set ablaze by the German minesweepers R-18 and R-19. Sørland sank with two of her six-man crew near the village of Skiphelle in Drøbak, as the first Norwegian merchant vessel lost during the invasion. 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 Sterlet and sank on the following day.

The fortress was subjected to heavy Luftwaffe bombing later on 9 April, to which the fortress could reply only with two Bofors 40-mm anti-aircraft guns and three machine guns at Seiersten battery, as well as another four machine guns at Håøya battery, but again there were no Norwegian casualties. Initially, four machine guns on the roof of the main battery also returned fire, but these had to be abandoned early in the German air attack. One of the two 40-mm guns became unserviceable after firing a mere 22 rounds, but the other kept firing until 12.00, although to little effect. After a break in the attacks from 12.00 to 13.30, during which time Lützow bombarded Hovedøya, the German bombers returned and strafed the remaining Norwegian anti-aircraft guns, forcing the crew to seek shelter in the nearby forest at around 14.00. In all, the fortress was subjected to nearly nine hours of air attack, during which time around 500 bombs, ranging in weight from 110 to 441 lb (20 to 200 kg), were dropped on Oscarsborg. Among the bombers which attacked Oscarsborg were 22 Junkers Ju 87R single-engined long-range dive-bombers of the Sturzkampfgeschwader 1 under the command of Hauptmann Paul-Werner Hozzel, operating from Kiel-Holtenau airport in northern Germany.

Although the German naval attack on Oslo had been thwarted by the actions of Oscarsborg, the city was seized later that day by forces airlifted into Fornebu airport. In light of the fall of the capital, and with news of German landings at the village of Son to the south of Drøbak, Eriksen decided that further fighting without adequate infantry support would be both useless and costly, and therefore agreed to a ceasefire in the evening of 9 April. The fortress was surrendered intact on the morning of 10 April.

The garrison of the main battery and at Håøya were treated separately from those captured in the mainland batteries, and were released a week after the battle. The soldiers and non-commissioned officers taken in the mainland batteries were released three days after the fortress’s surrender, while the officers were held as prisoners of war at the Fredriksten fortress. The reserve officers were released on 15 May, while the full-time officers were transferred to Grini prison camp and released in late May 1940.

In one of the more peculiar battles of the war, a fortification 100 years old, manned by raw recruits and pensioners, and armed with 40- to 50-year old weapons of German and Austro-Hungarian manufacture, had destroyed a ship so new that its crew was still finishing its training. Oscarsborg had fulfilled its mission and denied an invader access to the capital. Even though it and the country were ultimately defeated and occupied, the effects of delaying the German advance were immediate and considerable. On board Blücher were troops specially allocated to capture the king, the Norwegian cabinet, the Storting (Norwegian parliament) and the national gold reserve. The delay made it possible for all these to escape. On 9 April, the Storting was able to convene at Elverum and give the cabinet wide-ranging authorisation to govern until a Storting could again assemble. Thus, the Norwegian government was able to continue the defence of the country until it had been evacuated to exile in the UK on 7 June, with the Norwegian army laying down its arms on 10 June.