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 1890s Krupp guns and 40-year-old Whitehead torpedoes at near point-blank range. The Blücher sank by 07:23. The 12-hour delay saved King Haakon VII, the cabinet, and 50 tons of gold.

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 passing through has no room to manoeuvre. No space to dodge. In military terms, it’s a kill zone. On the morning of 9 April 1940, that is exactly what it became.

In the darkness before dawn, a column of German warships entered the Oslofjord. Their mission: reach Oslo, seize the capital, arrest King Haakon VII before the Norwegian government could flee. At the head of the column sailed the heavy cruiser Blücher, an Admiral Hipper-class warship launched 8 June 1937 at Deutsche Werke Kiel and commissioned 20 September 1939 ( Store norske leksikon records 14,050 tons standard displacement, 18,500 at full load). She had been in service barely six months. One of the newest warships in the German fleet.

The Blücher never reached Oslo. What happened in the Drøbak narrows over the next three hours is one of the most lopsided David-versus-Goliath moments of the Second World War, and the single most consequential military action in Norwegian history.

The fortress in the narrows

Oscarsborg Fortress sits on a small island in the middle of the Drøbak strait, roughly 35 kilometres south of Oslo. People have understood the strategic value of this chokepoint for centuries. The first fortifications went up in 1644 under Christian IV, King of Denmark-Norway. The modern fortress took shape between 1848 and 1853, and got its current name via royal decree in 1855, after King Oscar I. We pass the fortress regularly on our tours heading south through the fjord, and even from a distance you can feel why someone decided to put a gun battery there. The strait is absurdly narrow.

When I take the boat through the narrows past Oscarsborg, the fjord squeezes to about 500 metres. From the wheel you can see both shores without turning your head. A heavy cruiser doing 12 knots through this gap had no room to manoeuvre. The Krupp guns closed the gap at point-blank range; the torpedoes hit from below.

From the 1890s onward, Oscarsborg was armed with three 28-centimetre (11-inch) Krupp guns, among the most powerful coastal defence weapons of their era. The garrison named the three guns “Moses,” “Aaron,” and “Joshua.” They were old. By 1940, they had been in service for nearly half a century.

But the fortress held a secret. Between 1898 and 1901, an underwater torpedo battery had been constructed beneath the surface at Kopås, on the eastern shore of the strait (history detailed at Store norske leksikon). Three torpedo tunnels capable of firing six torpedoes without reloading, with nine stored and ready. The torpedoes themselves were 40-year-old Whitehead models manufactured in Austria-Hungary.

German military intelligence did not know this torpedo battery existed. The guns were old and on the charts. The torpedoes were invisible.

Operation Weserübung

In the early hours of 9 April 1940, Nazi Germany launched Operation Weserübung: the simultaneous invasion of Denmark and Norway (operational overview at Store norske leksikon). Denmark surrendered within hours. Norway was intended to follow the same script. A naval task force would sail up the Oslofjord under cover of darkness, dock at Oslo’s quayside before dawn, and present the Norwegian government with a fait accompli.

The invasion fleet entering the Oslofjord was formidable. The Blücher led the column, accompanied by the heavy cruiser Lützow, the light cruiser Emden, torpedo boats, and minesweepers. The ships carried approximately 1,000 troops of the 163rd Infantry Division, plus Gestapo officers and German administrative officials. Here’s the telling detail: these men were not soldiers expecting combat. They were bureaucrats carrying occupation plans, government seizure orders, and the paperwork needed to immediately establish German control over the Norwegian state. They packed filing cabinets. They expected a desk job, not a battle.

The plan depended on speed and surprise. Arrive before dawn. Dock at the quayside. Arrest the king. Seize the government. Control the treasury. By the time Norwegians woke up, it would be over.

“Either I will be decorated, or I will be court-martialed”

Oscarsborg’s commander was Colonel Birger Eriksen, a career officer who had spent decades in the Norwegian coastal artillery (biography: Store norske leksikon). His garrison that night was not an elite fighting force. Most of the men were reservists and recent conscripts. Many had never fired the old Krupp guns in live combat. Some had arrived at the fortress only days earlier.

Around 04:00, Norwegian lookouts detected approximately six ships approaching through the darkness. Eriksen faced a decision that no training exercise could prepare a man for. He had received no clear orders from the Norwegian high command. No confirmed intelligence about whether the approaching warships were German or British (the Royal Navy had also been threatening Norwegian neutrality). The ships were running dark, showing no flags.

If Eriksen opened fire on a friendly or neutral vessel, he could trigger a diplomatic catastrophe. If he held his fire and the ships were hostile, Oslo would fall before breakfast.

The colonel made his decision.

“Either I will be decorated, or I will be court-martialed. Fire!”

The Battle of Drøbak Sound: minute by minute

04:15–04:20. Norwegian searchlights snapped on, cutting white beams across the black water. The lead vessel was illuminated: a large warship, close enough that every detail of its superstructure was visible from the fortress walls.

04:21. Moses and Aaron spoke. The 28-centimetre guns of Oscarsborg’s main battery opened fire at near point-blank range. At the short distance of the Drøbak narrows, the ancient Krupp guns could not miss. The first shell struck high above the Blücher’s bridge, destroying the anti-aircraft gun command station. The second hit near the aircraft hangar amidships, igniting aviation fuel. A fire started that could not be controlled.

Simultaneously, the 15-centimetre guns of the Kopas battery on the eastern shore opened fire, scoring multiple hits along the cruiser’s hull and superstructure.

04:34. Then came the blow the Germans never expected. Two torpedoes from the concealed Kopas torpedo battery struck the Blücher below the waterline. The torpedo battery was commanded by Commander Andreas Anderssen, who lived in Drøbak and was serving as a temporary substitute because the regular commanding officer had gone on sick leave in March 1940. A local man filling in at the last possible moment fired the torpedoes that decided the battle. You really couldn’t make it up.

04:40. The German fleet began to withdraw. The Lützow, attempting to reverse course in the narrow strait, was struck by three 15-centimetre shells from the Kopas battery. One disabled the cruiser’s forward turret. The battery continued firing until the German ships disappeared into the mist at a range of approximately 3,000 metres.

06:22. After burning for nearly two hours, the Blücher capsized and sank in the Drøbak strait (time recorded by Store norske leksikon). Casualty estimates vary widely: SNL cites 700–1,000 dead; lokalhistoriewiki.no notes the figures span 320 to 1,000 of the roughly 2,202 men aboard.

Norwegian casualties: zero.

Reservists, conscripts, and antique weapons

The men who fired those guns were not commandos. They were civilians in uniform, called up days or weeks earlier. The Krupp guns dated from the 1890s. The torpedoes were Austro-Hungarian designs from the turn of the century. The torpedo battery was operated by a handful of men under a stand-in commander who happened to live nearby.

The Blücher was six months old. The torpedoes that sank her were forty years old. The guns were fifty. The fortress was nearly a hundred. These improvised defenders, using weapons older than themselves, sank one of Germany’s most advanced warships. Technology lost. Geography and courage won.

Twelve hours that saved a nation

The sinking of the Blücher and the repulse of the invasion fleet delayed the German capture of Oslo by several hours (the University of Oslo’s Norgeshistorie records that the delay gave the government, parliament, and the royal family time to evacuate the capital). That delay was the single most consequential military action in Norwegian history. During those hours, four things happened that changed everything.

King Haakon VII escaped. The king left Oslo by train, eventually reaching northern Norway and then exile in London. From there, he led the Norwegian government-in-exile throughout the war, preserving the constitutional continuity of the Norwegian state.

The government and parliament evacuated. Norway’s democratic institutions survived intact. The country never surrendered constitutionally. Unlike Denmark, which capitulated within hours, Norway maintained a legitimate government that continued to fight.

Fifty tons of gold were saved. On the morning of 9 April, as news of the invasion spread, officials at Norges Bank in Oslo began loading the nation’s gold reserves onto trucks. Approximately 50 tons of gold bars and coins (valued at the time at roughly 200 million Norwegian kroner) were transported north by a desperate combination of trucks, trains, and fishing boats. The gold stayed just ahead of advancing German forces, moving through Molde, then Tromsø, then across the sea to the Bank of England in London and later the Bank of Canada in Ottawa. Without the hours gained by the Blücher’s sinking, that evacuation would have been impossible. I think the fishing boat part is the detail that gets me. Gold bars on a fishing boat. That’s the kind of thing that sounds made up but isn’t.

Critical state documents were secured. Diplomatic archives, intelligence records, and government files were evacuated alongside the gold and the government, denying the occupiers the administrative tools for an immediate, seamless takeover.

Had the Blücher arrived at Oslo’s quayside as planned in the early morning darkness, the Gestapo officials aboard would have arrested the king and government before sunrise. Norway’s resistance would have been decapitated before it began. The five years of organised opposition that followed (the government-in-exile, the merchant fleet that served the Allied cause, the domestic resistance) would never have materialised.

A postwar military commission unanimously approved Eriksen’s decision to fire. He was decorated, not court-martialed.

The water between the shores

The Blücher carried not only naval personnel but also hundreds of army troops and administrative officials who had expected to step ashore at a peaceful quayside. When the ship capsized, men were trapped below decks or pulled under by the suction of the sinking hull. Others escaped into the fjord’s icy April waters and faced hypothermia as they swam several hundred metres toward shore. April water in the Oslofjord is around 4 to 6 degrees Celsius. At that temperature, you have maybe 15 minutes before your muscles stop cooperating.

Local Norwegian civilians from Drøbak and the surrounding communities watched the catastrophe from the shoreline. Some waded into the water to help German survivors struggling ashore. Others, grasping the implications of what they had just witnessed, immediately began preparations to evacuate or resist. Humanitarian rescue and the dawning realisation of invasion happened at the same time, on the same stretch of shore, in the same cold morning light.

Among the items lost with the Blücher were the German occupation plans, administrative documents, and equipment intended for the immediate establishment of a German-controlled Norwegian government. Their loss further complicated the occupation of Oslo when German forces finally entered the city later that day via aircraft and overland routes.

A war grave at the bottom of the fjord

The Blücher lies at the bottom of the Drøbak narrows at a depth of approximately 64 metres. In 2016, the Norwegian Directorate for Cultural Heritage designated the wreck as a protected war memorial. Diving the wreck without permission is forbidden, both out of respect for the hundreds of men still entombed aboard and to protect the site’s significance as cultural heritage.

The wreck also poses an ongoing environmental problem. By 1991, oil leakage from the ship’s fuel tanks had reached 50 litres per day. In October 1994, divers drilled holes in 133 tanks and recovered approximately 1,000 tons of oil (figures from lokalhistoriewiki.no). Some bunkers proved unreachable. Structural deterioration continues. Recent 3D photogrammetric scans have enabled authorities to track structural changes and model potential collapses. The Blücher is both a war memorial and an active environmental headache, which is a very Oslofjord combination of solemn history and practical worry.

The torpedo battery that would not retire

The postscript to the battle is almost as remarkable as the battle itself. After the war, the Kopas torpedo battery (the same installation that sank the Blücher) was not decommissioned. It was modernised in the 1980s with updated equipment and remained secretly operational throughout the Cold War.

The torpedo battery was finally deactivated on 1 January 1993. Its service life spanned 92 years, from its commissioning in 1901 through two world wars, a Nazi occupation, and the Cold War, into the post-Soviet era. That is one of the longest continuous operational records for any naval weapon installation in Europe.

The same weapon that had been a secret from German intelligence in 1940 remained a secret from Soviet intelligence for another five decades. The Drøbak narrows never stopped being defended.

Why was the sinking of the Blücher so important?

The sinking of the German heavy cruiser Blücher by Oscarsborg Fortress on 9 April 1940 delayed the Nazi capture of Oslo by twelve hours. That delay enabled King Haakon VII, the Norwegian government, and approximately 50 tons of gold reserves to escape the capital, preserving Norway’s democratic institutions and financing its resistance throughout the Second World War.

Remembering the battle

The Oslofjord Museum (Oslofjordmuseet) at Drøbak maintains a permanent exhibition called “Tales of the Blücher,” documenting the battle and its aftermath from both Norwegian and German perspectives. The Oscarsborg Fortress Museum tells the story from the defenders’ side, with the original guns and torpedo installations accessible to visitors.

Each year on 9 April, commemorative events are held at the fortress and in Drøbak. The Norwegian film The Battle of Oslo (2025) brought renewed public attention to the story, dramatising the events of that night for a new generation.

Can you visit Oscarsborg Fortress today?

Oscarsborg Fortress is open to the public and surprisingly easy to reach. A short ferry crosses from Drøbak to the fortress island throughout the summer season. Drøbak itself is roughly 40 kilometres south of Oslo, reachable by local bus in about an hour from the city centre.

On the fortress island, visitors can walk the ramparts, enter the torpedo battery tunnels, and stand beside the Krupp guns that opened fire on the Blücher. The Oscarsborg Fortress Museum is operated by the Norwegian Armed Forces Museum. Admission to the museum is free; you pay only for the ferry crossing.

Beyond the military history, Oscarsborg has become a cultural venue. Summer opera performances are staged in the fortress courtyard, with the natural stone walls providing both backdrop and acoustics. The fortress also operates as a hotel and conference centre, and it is one of the few places in Norway where you can sleep inside a 19th-century fortification overlooking the fjord. On a full-day tour, we can take you past the fortress and through the narrows where the Blücher went down. If you’re going to pick one day trip from Oslo that combines history, scenery, and a decent cafe lunch, this is the one I’d recommend.

The practical details: the ferry from Drøbak to Oscarsborg runs frequently during summer (roughly May through September). Check the Forsvarsbygg website for current schedules and special events. The fortress grounds are open for free exploration; guided tours are available in summer. Allow at least two to three hours to see the museum, torpedo battery, and gun positions properly.

The narrowest point of the fjord

The Oslofjord is roughly 100 kilometres long. For most of that distance, it is wide enough that the far shore is a smudge on the horizon. But at Drøbak, it squeezes down to a few hundred metres of open water between fortress walls. That geography has been decisive for centuries, from the first gun emplacements of 1644 to the Cold War torpedoes deactivated in 1993.

Passing through the Drøbak narrows on a boat, you can point to the exact positions of the guns that fired on the Blücher, the tunnel openings from which the fatal torpedoes were launched, and the stretch of water beneath which the wreck still lies. With a full day on the water, there is time to explore both the narrows and the outer fjord beyond. Few places in the world carry that much military history that close to the surface.

The Akershus Fortress that surrendered without a shot the same morning still stands 35 kilometres to the north of the Oslofjordnarrows where the Blücher went down. The ferry to Oscarsborg runs every half hour in summer. Bring a jacket.

Sources

• Store norske leksikon — Senkingen av Blücher (time of sinking, casualty range, command sequence).
• Store norske leksikon — Blücher (specifications, Deutsche Werke Kiel construction, launch and commissioning dates).
• Store norske leksikon — Oscarsborg festning (Krupp gun installation, Kopås torpedo battery, fortress naming under Oscar I in 1855, deactivation timeline).
• Store norske leksikon — Birger Eriksen (commander biography and the “decorated or court-martialed” quote).
• Store norske leksikon — Weserübung: Tysklands angrep på Norge i 1940 (operational plan, force composition, Oslo group fate).
• Norgeshistorie (Universitetet i Oslo) — Overfallet (12-hour delay, evacuation of king and government, gold reserves).
• Lokalhistoriewiki — Senkningen av Blücher (1994 oil-recovery operation: 133 tanks, ~1,000 tons; wreck depth; environmental status).
• Mispelkamp, “Avoidable Loss: The Saga of the Blücher” — The Northern Mariner VI:3 (1996) (peer-reviewed analysis of the loss from the German command perspective).
• Forsvarsbygg — Oscarsborg festning (current visitor information; managed as a national heritage fortress).