It took place during Operation Weserübung, when the Germans launched an amphibious assault on Karljohansvern, the Norwegian naval base at Horten.

After being initially repulsed by Norwegian naval units, German troops headed overland to outflank the base, forcing it to capitulate.

On the morning of 9 April two operational ships were in the harbour: the minelayer Olav Tryggvason (present for repairs[1]) and the minesweeper Rauma.

[2] Also present at the base were the dilapidated coastal defence ships Tordenskjold and Harald Haarfagre which, being unarmed, decommissioned training and depot vessels, took no part in the fighting.

[5] Shortly after midnight Admiral Smith-Johansen informed the captain of Olav Tryggvason, T. Briseid, that there had been a battle at the mouth of the Oslofjord, and it was likely that the enemy ships had breached the outer fortifications.

[8] Captain Grundmann, chief officer of two R-boats,[9] seeking to establish communication with the Norwegian naval staff, managed to contact Admiral Smith-Johansen.

Bluffing, he declared that a large German force had seized the town of Horten and that it would be bombarded if the admiral did not surrender within half an hour.

Olav Tryggvason was renamed Brummer;[1] Rauma became Kamerun;[citation needed] and Tordenskjold and Harald Haarfagre were transformed into floating anti-aircraft batteries as Nymphe and Thetis, respectively.