The Battle of Drøbak Sound took place in Drøbak Sound, the northernmost part of the outer Oslofjord in south-eastern Norway, on 9 April 1940. It marked the end of the "Phoney War" and the beginning of World War II in Western Europe.
A small fleet of the Nazi German navy (the Kriegsmarine) led by the heavy cruiser Blücher was dispatched up the Oslofjord to begin the German invasion of Norway, to seize the Norwegian capital Oslo and capture King Haakon VII and his government. The fleet was engaged in the fjord by Oscarsborg Fortress, an aging coastal installation near Drøbak, that had been relegated to training coastal artillery servicemen – leading the Germans to disregard its defensive value. However, unbeknownst to German military intelligence, the fortress' most powerful weapon was a torpedo battery, which would be used to great effect against the German invaders.
The fortress' armaments worked flawlessly despite their age, sinking the Blücher in the fjord and forcing the rest of the German fleet to withdraw. The loss of the German flagship, which carried most of the troops and Gestapo agents intended to occupy Oslo, delayed the German occupation long enough for King Haakon and his government to escape from the capital.
Before the battle
Norway
Oscarsborg Fortress was commanded by 64-year-old Oberst (Colonel) Birger Eriksen. He had not received clear orders nor any information on whether the approaching warships were German or Allied. He knew that Norway was officially neutral but that the government was inclined to side with the British if Norway became a belligerent.
The fortress is located on two islets adjacent to the southeastern shore of the island of Håøya, located in Oslofjord about south of Norway's capital Oslo, which is at the head of the fjord that bears its name. The two islets and Håøya are separated by a channel some wide. The islets are together known as the , with most of the fortifications and armaments on South Kaholmen (). Other fortifications are on Håøya and both sides of Oslofjord; an underwater barrier blocks the passage of large vessels between Håøya and the western shore of the fjord. The wide passage between the eastern shores of the Kaholmene and the eastern shore of Oslofjord is the northern end of Drøbak Sound.
The main observation and command station for the battery was on the southern end of Håøya, but due to the unique circumstances in 1940, Eriksen commanded from the backup station on South Kaholmen () beside the main battery.
Apart from the officers and non-commissioned officers, most of the remaining garrison consisted of 450 fresh troops conscripted only a week earlier on 2 April. The naval mines were not deployed; their deployment was scheduled for a few days after 9 April as a training exercise for the new recruits.
The main battery consisted of three aging guns (made at the turn of the century by the German armaments firm Krupp) called Moses, AronHauge 1995: 35 and Josva. There were enough trained gunners for one gun. They were split between two guns and assisted by non-combatant privates, including cooks. All three guns were loaded with live high-explosive shells.
The fortress' torpedo battery, on the southeastern side of the islet of North Kaholmen () (whose southern shore is only north of South Kaholmen), was armed with Whitehead torpedoes that had been manufactured at the turn of the century in Austria-Hungary. The weapons had been practice-launched over 200 times; despite doubts, they functioned properly during the battle. The battery had three launch tunnels opening below the surface. There were six torpedoes ready to launch, and another three were available as reloads.
The torpedo battery was commanded by Kommandørkaptein (Commander Junior Grade) , who lived in Drøbak. Late on 8 April, during the action in the Oslofjord further south near the island of , Eriksen ordered Anderssen, who was a few weeks away from his 61st birthday, to the battery; Anderssen donned his old uniform and was transported the short distance across the sound to the battery by boat.Hansen 2005: 49 Anderssen was familiar with the post; he first served at the battery in 1909 and he retired in 1927 as its commander. Anderssen was recalled to active duty in March 1940 as a temporary substitute for the battery’s commanding officer, who was on sick leave.Hansen 2005: 48
Germany The objective of the assaulting naval force – designated Kampfgruppe ("fighting group", or combat group) 5 by the Germans – was to land troops in Oslo who were to capture the King of Norway, Haakon VII, and the Norwegian government. Kampfgruppe 5 was led by the heavy cruiser German cruiser Blücher, a new warship with an inexperienced crew. Those onboard included Generalmajor Erwin Engelbrecht, Admiral Oskar Kummetz,Arneberg 1989: 62, 65. and a special unit for capturing the king. Also in the fleet was Lützow, another heavy cruiser, albeit smaller than Blücher; and Emden, a light cruiser. The three cruisers were supported by several smaller vessels, including the torpedo boats , and .
The Germans underestimated the operational value of the Norwegian coastal fortifications; the fortifications were old and used for training. The Germans did not know of the Oscarsborg torpedo battery.
Battle
The main battery fires At 4:21a.m. on 9 April, Eriksen ordered the main battery to fire on Blücher, the lead ship of the German flotilla heading to Oslo. When his order was questioned, Eriksen replied: "Either I will be decorated or I will be court-martialled. Fire!" Firing without warning shots violated the pre-war Norwegian rules of engagement. The German ships had already received warning- and live shots from outlying fortifications on Rauøy; Eriksen later used this to justify his decision to consider the ships to be hostile.
Two guns, Moses and Aron, each fired one high-explosive shell.Hauge 1995: 35 The first shell struck the front of the mainmastRibsskog 1998: 50 and set the midship area up to the fore mast on fire;Ribsskog 1998: 50–51 this detonated a magazine containing stores for the Arado Ar 196 reconnaissance floatplanes – oil cans, smoke dispensers, incendiary bombs, aircraft bombs and depth charges. Shortly after the first hit, the second shell hit the base of one of the forward gun turrets, throwing debris from it overboard and igniting further fires;Ribsskog 1998: 51 this put the ship's main guns out of action by disabling their electrical power. The unmanned third gun, Josva, was not fired. No further shells were fired; the guns could not be reloaded in time with the untrained gunners.
The secondary batteries fire Next, the secondary Norwegian coastal batteries on the mainland east of the islets opened fire on Blücher. The weapons ranged from two small guns at Husvik (at the narrowest point of Drøbak Sound, less than from South Kaholmen), intended to protect the fortress' missing mine barrier; to the three guns of the Kopås Battery south of Husvik Battery. The 57 mm guns targeted the cruiser's superstructure and anti-aircraft (AA) weapons,Fjeld2 1999:36 and partially suppressed German return fire. Blücher continued to sail slowly northward. It passed close enough to fire on the Husvik battery with light AA guns. The Norwegians abandoned Husvik; its main building caught fire but there were no casualties. In all, the cruiser was hit by thirteen 15 cm and around thirty 57 mm shells. One 15 cm hit from Kopås disabled the steering gear; the ship avoided grounding by steering with its engines. Shell fragments disabled the firefighting system.Fjeld2 1999:36
The fortress' gun batteries had been in action for only five to seven minutes. Return fire from Blüchers light battery was ineffective due to excessive elevation.Binder 2001: 77
At this point, the voices of Germans on the cruiser became audible to the Norwegians, alerting the latter of their opponent's identity;Berg 1997: 12–13Ribsskog 1998:45–50Grimnes 1990: 7–8 according to the Norwegians, the Germans began to sing Deutschlandlied, the German national anthem.Hauge 1995: 36Tamelander&Zetterling 2001: 87Berg 1997: 13 The Norwegian minesweeper HNoMS Otra had identified the intruders as Germans earlier and communicated this to the Horten naval base at 04:10. Norwegian communications problems delayed its delivery to Oscarsborg; Eriksen received it at 04:35.Berg 1997: 10 There was then "a dead silence on board the whole ship, no movement whatsoever was identified".
The torpedo battery attacks Blücher was crippled but remained afloat. It continued to move north, which brought it within of Oscarsborg's torpedo battery on North Kaholmen.Hauge 1995: 37 Anderssen launched the first torpedo at about 04:30. The target's speed was slightly overestimated, and the torpedo struck near the ship's forward turret, causing inconsequential damage. The aim was corrected for the second torpedo; it struck amidships in the same area as the first 28 cm shell hit. The hit disabled the engines,Hauge 1995: 38 blew open bulkheads and caused flooding. The ship continued to burn.
The fortress' third torpedo tube was not used, being kept in readiness for attacking further targets. The other tubes were reloaded.
Blücher sinks
Blücher anchored near the , a group of four islets east of Håøya and about north of the Kaholmene; and out of the field of fire of the fort's guns. The crew attempted to fight the uncontrolled fires. Torpedoes were launched from the ship to prevent their warheads' detonation by the fires. At 05:30,Hauge 1995: 38 the fires detonated a midships magazine for the Flak guns, blowing a hole in the ship's side,Williamson 2003: 33–34Tamelander&Zetterling 2001: 88 rupturing the bulkheads between the boiler rooms, and causing further fires by breaching fuel tanks.
Blücher sank bow-first at 06:22, capsizing to port. Two thousand German sailors and soldiers were in the freezing water. Hundreds died when floating fuel oil caught fire.
Approximately 1,400 Germans survived and 650–800 died. Some 1,200 came ashore at Frogn near Drøbak.Hansen 2005: 71 Of these, 550 were captured by His Majesty the King's Guard, 4th Company, commanded by Kaptein (Captain) A. J. T. Petersson. The guardsmen were supposed to take all the Germans prisoner, but mainly focused on treating casualties.Hansen 2005: 71 Around 1,000 Germans, including Engelbrecht and Kummetz, were eventually moved to a nearby farm and placed under light guard. The prisoners were not interrogated, and were effectively freed when the Norwegians withdrew by 18:30. Engelbrecht and Kummetz reached OsloArneberg 1989: 62, 65 at 22:00, establishing themselves in the Hotel Continental, and occupying the capital with their remaining troops.Arneberg 1989: 65Hansen 2005: 72 Norwegian wounded and many German wounded were treated by the Royal Norwegian Navy Hospital at the Asgården summer hotel in Åsgårdstrand; the hospital had been evacuated from Horten at midnight on 8 April.Sivertsen 2001: 78
Remaining ships retreat When Blücher was hit by the torpedoes, the commander of Lützow – unaware of the torpedo battery – assumed the presence of mines. At 04:40, the Germans decided to withdraw and land their troops out of range of the Oscarsborg guns. The attack on Oslo would continue by a landward advance up the Oslofjord.Grimnes 1990: 14
The retreating Lützow was hit by three 15 cm shells from the Kopås battery, which disabled the cruiser's forward 28 cm turret. The guns at Kopås continued to fire until the German ships disappeared into the mist at a range of around . Lützows aft turret bombarded the Norwegians from a range of down the fjord.