Battle of Drøbak Sound

Norway Oscarsborg Fortress was commanded by 64-year-old Oberst (Colonel) Birger Eriksen. He had not received clear orders and no 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' main command station and battery were on Håøya island north-west of South Kaholmen (.) Due to the unique circumstances in 1940, Eriksen commanded from the backup station at South Kaholmen, east of the main battery. Apart from the officers and NCOs, most of the remaining garrison consisted of 450 fresh troops conscripted on 2 April. The naval mines were not deployed; deployment was scheduled for a few days after 9 April as a recruit training exercise. The main battery consisted of three 28 cm MRK L/35| Krupp guns called Moses, Aron and Josva. There were enough trained gunners for one gun. They were split between two guns and assisted by non-combatant privates, Anderssen was familiar with the post; he first served at the battery in 1909 and was its commander by the time he retired in 1927. Anderssen was recalled to active duty in March. Germany The objective of the German naval force was to capture the Norwegian capital, Oslo, King Haakon VII, and the Norwegian government. It was led by the cruiser Blücher, a new warship with an inexperienced crew. Those onboard included Generalmajor Erwin Engelbrecht, Admiral Oskar Kummetz and a special unit for capturing the king. ==Battle==

, the commander of Oscarsborg, 9 April 1940 and the fortress of Oscarsborg The main battery fires main battery guns at Oscarsborg At 4:21 a.m. on 9 April, Eriksen ordered the main battery to fire on Blücher, the lead ship of the German flotilla heading to Oslo. Firing without warning shots violated the pre-war Norwegian rules of engagement. The first shell struck the front of the mainmast and set the midship area up to the fore mast on fire; this detonated a magazine containing stores for the Arado Ar 196 reconnaissance floatplanes – oil cans, smoke dispensers, incendiary bombs, aircraft bombs and depth charges. this put the ship's main guns out of action by disabling their electrical power. 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. 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. At this point, Germans voices on the cruiser became audible to the Norwegians, alerting the latter of their opponent's identity; according to the Norwegians, the Germans began to sing Deutschlandlied, the German national anthem. 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. There was then "a dead silence on board the whole ship, no movement whatsoever was identified". Anderssen fired the first torpedo at about 04:30. The target's speed was slightly overestimated, and the torpedo struck near the ship's forward turret and caused inconsequential damage. The aim was corrected for the second torpedo; it struck amidship in the same area as the first 28 cm shell hit. blew open bulkheads and caused flooding. The ship continued to burn. the fires detonated a midship magazine for the Flak guns, blowing a hole in the ship's side ruptured the bulkheads between the boiler rooms, and caused further fires by opening fuel tanks. 550 were captured by His Majesty the King's Guard, 4th Company, commanded by Kaptein (Captain) A. J. T. Petersson. 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 Oslo at 22:00, establishing themselves in the Hotel Continental, and occupying the capital with their remaining troops. 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. ==Remaining ships retreat==

When Blücher was hit by the torpedoes, the commander of the heavy cruiser Lützow – unaware of the torpedo battery – assumed the presence of mines. At 4 h 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. The retreating Lützow was hit by three 15 cm shells from the Kopås battery, which disabled the cruiser's forward 28 cm turret. ==Luftwaffe bombing==

The fortress was heavily bombed for nearly nine hours and four machine guns at the main battery. The air attacks included 22 long-range Junkers Ju 87R "Stuka" dive bombers of Sturzkampfgeschwader 1, commanded by Hauptmann Paul-Werner Hozzel, operating from Kiel-Holtenau airport in northern Germany. Around 500 bombs — ranging from in size — were dropped. ==Surrender==

The Norwegian situation continued to deteriorate. The Germans took Oslo later in the day with troops airlifted into Fornebu Airport, and additional landings occurred at the village of Son south of Drøbak. The garrison went into captivity. The enlisted and NCOs of the secondary batteries were released three days later; those of the main battery a week later. The officers were initially taken to Fredriksten Fortress, with the reservists being released on 15 May. Regular officers were moved again to the Grini detention camp and released in late May 1940. ==Aftermath==

The destruction of Blücher by the Oscarsborg Fortress and the withdrawal of the German naval force caused a significant delay in the German capture of Oslo. The delay allowed King Haakon VII, Prime Minister Johan Nygaardsvold's government, the parliament, and the gold reserve to be evacuated. As the Norwegians retreated, the government was granted wartime emergency powers by the king and parliament; the affirmations included the Elverum Authorization of 9 April. The government maintained this legitimacy when it became a government-in-exile in the United Kingdom on 7 June shortly before the Norwegian Army surrendered on 10 June. ==Media adaptation==

The battle is depicted in the 2016 feature film ''The King's Choice and the 2025 Norwegian historical war film The Battle of Oslo'', by Daniel Fahre. == See also ==