The attack on Oslo was a key component of the German invasion of Norway of 9 April 1940, and saw the only real setback suffered by the Germans on that day. The Germans had planned a two-pronged assault on Oslo. A naval force would attack up Oslo Fjord, carrying two battalions from the 163rd Division. At the same time paratroops would land at Fornebu airfield. They would then be reinforced by an airlift of troops. The main aim of the attack was to capture the Norwegian government and King Haakon VII, to prevent them organising any resistance to the German invasion.
Neither attack went according to plan. The naval force included the pocket battleship Lützow, the new heavy cruiser Blücher and the light cruiser Emden, under the command of Vice-Admiral Kummetz. They left Kiel on the morning of 8 April, reaching the entrance to Oslo Fjord at midnight. The outer defences of the fjord were quickly neutralised, and the fleet moved on towards the most dangerous part of the fjord. This was the Dröbek narrows, guarded by the fortress at Oscarborg fortress, armed with 5.9in and 11in guns. The fort was commanded by Colonel Birger Erikson, then aged 65.
Erikson had not been ordered to go onto a war footing, and his standing orders were to fire warning shots before opening fire, but he decided that any warship that had reached this far had already received warning shots, and so at 4.20am opened fire with two 11in guns, firing one shell from each gun. The Blücher was hit and badly damaged by the two shells. She then passed in front of the Norwegian shore based torpedoes, and was hit twice. Two hours later she capsized and sank with the loss of around one third of the 2,000 men onboard at the time.
The rest of the German naval force turned back soon after the Blücher was first hit. The remaining troops were landed ten miles further down the fjord, and began to advance towards Oscarborg. The Blücher had been carrying the troops that had been given the task of seizing the Norwegian government at Oslo. Their absence from the city on 9 April allowed the Norwegian parliament and King Haakon VII to escape from the German trap, taking with them the Norwegian gold reserves.
The aerial assault also ran into problems. Fog prevented the initial paratrooper attack, and so the transport planes were ordered back to base. One group disobeyed that order, landing at Fornebu airfield at Oslo. They came under fire from the defenders of the airfield, but were soon able to take control. Reinforcements were flown in from a recently captured airbase in Denmark, and by the end of the day most of the city was in German hands.
The setback at Oslo had a serious impact on the German campaign in Norway. The Norwegian government managed to broadcast orders for mobilisation before the Oslo broadcasting station fell into German hands. The king and the government were able to escape from German pursuers, eventually reaching safety with the British. They were then evacuated from Andalsnes, setting up a Norwegian government in exile in London.