The Sinking of the Blücher: The Night Oscarsborg Saved Norway

On 9 April 1940 the German heavy cruiser Blücher led the invasion fleet up the Oslofjord. At Drøbak the channel narrows to 500 metres. Norwegian reservists at Oscarsborg Fortress fired Krupp guns from the 1890s and forty-year-old Whitehead torpedoes at close range. She capsized and sank at 06:22, and the delay got King Haakon VII, the cabinet and 50 tonnes of gold out of the capital.

The Oslofjord narrows to barely 500 metres at Drøbak (DROO-bahk). Stand on the fortress ramparts and you can see both shores without turning your head. A ship in there has no room to manoeuvre, which is the entire reason there is a fortress in the middle of it.

In the darkness before dawn on 9 April 1940, a column of German warships came up the fjord with orders to take Oslo and arrest King Haakon VII before the government could leave. At the head of it sailed Blücher, an Admiral Hipper-class heavy cruiser launched at Deutsche Werke Kiel on 8 June 1937. Store norske leksikon records her as completed on 30 September 1939, displacing 14,050 tonnes standard and 18,500 at full load. She had been a finished warship for six months.

Blücher never reached Oslo. She stopped 35 kilometres short of the quay she was meant to dock at, in a strait too narrow to turn around in.

The fortress in the narrows

Oscarsborg occupies two small islands in the middle of the Drøbak strait, roughly 35 kilometres south of Oslo. The chokepoint has been fortified since 1644, when Christian IV of Denmark-Norway put the first batteries here. The fortress took its present shape between 1845 and 1855 and received its name by royal decree that year, after King Oscar I (Store norske leksikon on the fortress). We pass it on tours heading south, and from the water nobody needs the strategy explained.

The main battery was three 28-centimetre Krupp guns, mounted in 1892 and 1893, firing shells of 345 kilograms. The garrison named them Moses, Aaron and Joshua. Up on the mainland shore above Drøbak stood a second battery at Kopås, three 15-centimetre Armstrong guns built between 1898 and 1900 (gun data from Wikipedia's articles on Oscarsborg festning and Kopåsbatteriet).

The installation that decided the morning was under water. Off North Kaholmen, the northern of the two fortress islands, ran a torpedo battery built between 1898 and 1901 and put into service on 15 July 1901: three tunnels that could send six Whitehead torpedoes away without reloading, with nine more in store. Norway had exactly two of these underwater batteries, this one and the battery guarding Bergen from Kvarven. German intelligence had the old guns on its charts and nothing at all on the torpedo tunnels.

Coming through the narrows past Oscarsborg, the fjord squeezes to about 500 metres. From the wheel you see both shores without turning your head. A heavy cruiser in there has nowhere to go. The big guns were on the island a few hundred metres off, the Armstrong guns were up on the Drøbak side, and the torpedo tubes were under the water in between.

Operation Weserübung

In the early hours of 9 April 1940, Germany launched Operation Weserübung, the simultaneous invasion of Denmark and Norway (Store norske leksikon on the operation). Denmark capitulated within hours, and Norway was supposed to follow the same script: a naval force up the Oslofjord under cover of darkness, alongside in Oslo before dawn, the Norwegian government handed a fait accompli.

The Oslo group was built for exactly that. Blücher led, with the heavy cruiser Lützow, the light cruiser Emden, torpedo boats and minesweepers behind her. Aboard were roughly 1,000 troops of the 163rd Infantry Division, Gestapo officers and German administrators carrying occupation plans, seizure orders and the paperwork for running the Norwegian state from that morning onwards. They had brought filing cabinets.

"Either I will be court-martialed, or I will be a war hero"

Oscarsborg's commander was Colonel Birger Eriksen, decades into a career in the Norwegian coastal artillery (biography at Store norske leksikon). His garrison was not an elite unit. Most of the men were reservists and recent conscripts, some of whom had arrived days earlier, and few of them had fired the Krupp guns in anger.

Around 04:00 the lookouts reported some six ships coming up the fjord, running dark and showing no flags. Eriksen had no clear orders from high command and no confirmation of whose ships they were; the Royal Navy had been pressing on Norwegian neutrality that winter too. Firing on a British or neutral vessel meant a diplomatic disaster. Holding fire, if the ships were German, meant Oslo taken before breakfast.

Store norske leksikon records the words he is said to have used:

"Either I will be court-martialed, or I will be a war hero. Fire!"

The Battle of Drøbak Sound, minute by minute

00:07. Seven minutes into the day, the Kopås battery above Drøbak was manned and ready.

Around 04:00. Norwegian lookouts picked up the column in the dark. Searchlights caught the lead ship close enough to read the detail of her superstructure from the fortress wall.

04:21. Eriksen gave the order and the main battery fired (time recorded by Store norske leksikon). Moses and Aaron got off one shell each, and that was the entire contribution of the 28-centimetre battery: the third gun had no crew to work it, and the men on the other two were inexperienced. At that range both rounds hit. The first struck high, above the bridge, and destroyed the anti-aircraft command station. The second landed near the aircraft hangar amidships and set the aviation fuel alight. That fire was never brought under control.

04:21 onwards. The 15-centimetre Armstrong guns at Kopås opened up at the same moment and kept working, putting somewhere around 22 to 23 rounds into Blücher and Lützow over the course of the action. Smaller batteries at Husvik, Nesset and Seiersten fired as well.

04:34. Two torpedoes from the North Kaholmen battery hit below the waterline. The officer who fired them was Commander Andreas Anderssen, retired, who had run that same battery from 1910 to 1916 and again from 1919 to 1928, and who now lived in Drøbak as a manager at a coal firm. He was standing in because the regular commanding officer had gone on sick leave in March (Wikipedia's biography of Anderssen).

04:40. The German force turned back. Lützow, reversing out through the narrows, took three 15-centimetre shells from Kopås, one of which disabled her forward turret. The battery kept firing until the ships were lost in the mist at about 3,000 metres.

06:22. After burning for close to two hours, Blücher capsized and sank in the strait. Store norske leksikon puts the dead at 700 to 1,000; lokalhistoriewiki.no notes estimates ranging from 320 to 1,000 out of roughly 2,202 men aboard. Not one Norwegian was killed.

Old guns against a new ship

The shooting had lasted about twenty minutes. The ship was six months old. The torpedoes that opened her hull were forty years old, the guns had stood in their emplacements since 1892, and the men working them were civilians in uniform who had been called up weeks or days before. Two shells and two torpedoes, fired by a garrison of reservists and a retired officer on loan from a coal firm, took out one of the newest warships in the German navy.

What the delay bought

Losing Blücher and turning the fleet back delayed the German advance into Oslo by several hours. The University of Oslo's Norgeshistorie records that those hours gave the government, the parliament and the royal family time to get out of the capital.

King Haakon VII left Oslo by train, reached northern Norway and went on to exile in London, where he headed the Norwegian government-in-exile for the rest of the war. The cabinet and the Storting evacuated with him, which is why Norway never surrendered constitutionally the way Denmark had done that same morning.

The gold went the same way. As word of the invasion spread, staff at Norges Bank loaded the country's reserves onto trucks: roughly 50 tonnes of bars and coin, worth around 200 million kroner at the time, moved north by truck, train and fishing boat through Molde and Tromsø, then across the sea to the Bank of England and later the Bank of Canada. Diplomatic archives and government files travelled with the convoy.

The German occupation plans went down with the ship, along with the office equipment meant to set up a German-run Norwegian administration the same day. When German troops finally entered Oslo that afternoon, by air and overland, they arrived without any of it. Had Blücher come alongside before sunrise as planned, the Gestapo officers aboard would have had the king and the cabinet in custody by breakfast.

Eriksen retired in November 1940. After the war he was awarded the War Cross with Sword, the Participation Medal with rosette and the French Croix de guerre, and was made an officer of the Légion d'honneur. The court martial never came.

The water between the shores

Blücher was carrying hundreds of army troops and officials who had expected to walk down a gangway onto a quiet quayside. When she rolled over, men were trapped below decks or pulled down with the hull. Those who got clear had several hundred metres to swim. The Oslofjord in April sits at four to six degrees, and at that temperature you have perhaps fifteen minutes before your muscles stop answering.

People from Drøbak and the farms around it watched from the shoreline. Some waded in and pulled German survivors out of the water. Others went home and started packing.

A war grave at the bottom of the fjord

The wreck lies on the bottom of the Drøbak narrows at around 90 metres, almost upside down, the way she tipped in the last seconds before she went under (depth from lokalhistoriewiki.no). Since 2016 the Norwegian Directorate for Cultural Heritage has protected the site as a war memorial. Diving without permission is forbidden, out of respect for the men still inside the hull and to protect the wreck itself.

She is also an environmental problem. By 1991 her fuel tanks were leaking 50 litres a day. In October 1994 divers drilled into 133 tanks and recovered roughly 1,000 tonnes of oil, though some bunkers could not be reached and oil remains in the hull. The structure keeps deteriorating, and 3D photogrammetric surveys let the authorities track the changes and model how she might collapse.

From the surface there is nothing to see, just the water between Oscarsborg and the Drøbak shore. The position is known precisely, and any boat heading south through the narrows, including our full-day tour, passes directly over her.

The torpedo battery stayed in service until 1993

The installation that fired those two torpedoes was never decommissioned after the war. It was modernised in the 1980s and kept operational, and classified, through the Cold War. Deactivation came on 1 January 1993, ninety-two years after it entered service in 1901. It had been a secret from German intelligence in 1940 and stayed a military secret for another five decades.

Why was the sinking of the Blücher so important?

Blücher was carrying the officers who were supposed to arrest the Norwegian king and government on the morning of 9 April 1940, and sinking her delayed the German takeover of Oslo by several hours. In those hours King Haakon VII, the cabinet, the Storting and about 50 tonnes of gold reserves left the capital. Norway spent the war with a legitimate government abroad and the money to run it.

Remembering the battle

Oscarsborg fortress tells the story from the defenders' position, with the original guns and the torpedo installation open to visitors. Check the fortress pages for current museum opening hours before you go. Commemorations are held at the fortress and in Drøbak every 9 April.

Can you visit Oscarsborg Fortress today?

Yes, and it takes very little effort. A ferry crosses from Drøbak to the fortress island through the summer season, about five minutes each way. Drøbak is roughly 40 kilometres south of Oslo by road and about an hour from the city centre by bus.

On the island you can walk the ramparts, go into the torpedo battery tunnels on North Kaholmen and stand beside the Krupp guns that opened fire on Blücher. The museum is run by the Norwegian Armed Forces Museums and admission is free. You pay for the ferry crossing. Allow two to three hours. In summer the fortress courtyard is used for opera, and the fortress runs as a hotel and conference centre, so you can sleep inside a 19th-century fortification with the fjord outside the window.

From the water you can point at the gun positions, the tunnel mouths the torpedoes came out of and the stretch of surface the wreck lies under. With a full day on the water there is time for the narrows and the outer fjord in one trip. Thirty-five kilometres north, Akershus Fortress surrendered without firing a shot the same morning. Bring a jacket. The strait funnels wind.

Ferry times, museum hours and the opera season change through the year. Check the Forsvarsbygg ferry timetable and the fortress pages before you travel. Last reviewed: July 2026.

Sources

• Store norske leksikon: Senkingen av Blücher (04:21 order to fire, 06:22 sinking, torpedo battery at Nordre Kaholmen, 700–1,000 dead, supporting fire from Kopås, Husvik, Nesset and Seiersten).
• Store norske leksikon: Blücher (launched at Deutsche Werke Kiel, completed 30 September 1939, displacement).
• Store norske leksikon: Oscarsborg festning (fortress history, main battery, naming under Oscar I in 1855, torpedo battery timeline).
• Store norske leksikon: Birger Eriksen (biography, the quote as SNL reports it, decorations after the war, retirement in November 1940).
• Store norske leksikon: Weserübung, Tysklands angrep på Norge i 1940 (operational plan, force composition).
• Norgeshistorie (Universitetet i Oslo): Overfallet (delay of several hours, evacuation of king, government and gold reserves).
• Lokalhistoriewiki: Senkningen av Blücher (wreck at around 90 metres and almost inverted, 1994 oil recovery of 133 tanks and ~1,000 tonnes, casualty estimates from 320).
• Wikipedia: Oscarsborg festning (three 28 cm Krupp guns named Moses, Aron and Josva, mounted 1892–93, 345 kg shells, one round each from the two crewed guns).
• Wikipedia: Kopåsbatteriet (three 15 cm Armstrong guns built 1898–1900, battery manned at 00:07, 22–23 rounds fired).
• Wikipedia: Andreas Anderssen (battery command 1910–1916 and 1919–1928, coal-firm posting in Drøbak, stand-in command in April 1940).
• Mispelkamp, "Avoidable Loss: The Saga of the Blücher", The Northern Mariner VI:3 (1996) (peer-reviewed analysis from the German command perspective).
• Forsvarsbygg: Oscarsborg festning (current visitor information).