During a short exchange that resulted in the first Norwegian casualty of the war, the Germans managed to push through down to Drøbak Sound, where they would face more determined resistance.

The outer parts of the fjord were guarded by the Oslofjord Fortress, established in 1914, made up of smaller forts at Bolærne and Rauøy.

Of the 11 guard ships operating out of Tønsberg, HNoMS Pol III would be tasked with patrolling the Oslofjord on the night of 8 April.

[2] Shortly after 23:00 on 8 April, Lieutenant Hans Bergan of Pol III spotted the anonymous invasion force at the mouth of the Oslofjord.

[3] Captain Leif Welding-Olsen of Pol III had a short conversation with the commander of the German boat, Kapitanleutnänt Siegfried Strelow, who demanded his surrender, to which he refused.

Rauøy forwarded Captain Oslen's original warning down to Oscarsborg Fortress, which was able to halt the Germans in the Battle of Drøbak Sound, sinking Blücher and severely damaging Lützow.

Minesweepers and two torpedo boats detached from the main force would land troops in the area the next morning, and the Norwegians would withdraw from their forts over the next few days.