The Battle of Drøbak Sound took place in Drøbak Sound, the northernmost part of the outer Oslofjord in southern Norway, on 9 April 1940. It marked the end of the "Phoney War" and the beginning of World War II in Western Europe.
A German fleet led by the cruiser Blücher was dispatched up the Oslofjord to begin the German invasion of Norway, with the objective of seizing the Norwegian capital of Oslo and capturing King Haakon VII and his government. The fleet was engaged in the fjord by Oscarsborg Fortress, an aging coastal installation near Drøbak, that had been relegated to training coastal artillery servicemen, leading the Germans to disregard its defensive value. However, unbeknownst to German military intelligence, the fortress' most powerful weapon was a torpedo battery, which would be used to great effect against the German invaders.
The fortress' armaments worked flawlessly despite their age, sinking the Blücher in the sound and forcing the German fleet to fall back. The loss of the German flagship, which carried most of the troops and Gestapo agents intended to occupy Oslo, delayed the German occupation long enough for King Haakon VII and his government to escape from the capital.
Before the battle
As the political situation was chaotic, the 64-year-old commander, Oberst (Colonel) Birger Eriksen, had not received any clear orders and had received no notice as to whether the approaching warships were German or Allied. He was well aware that Norway was officially neutral, but that the government was inclined to side with the British in case of direct Norwegian involvement in the war.
Apart from the officers and NCOs, almost all the soldiers manning the fortress were fresh recruits, having only been conscripted seven days before, on 2 April. Because of the influx of 450 fresh recruits, the fortress' naval mines were not deployed on 9 April. Part of the recruits' training was to lay the mine barrier, a process planned for a few days later.
Torpedo battery
The commander of the torpedo battery at Oscarsborg had at the time of the battle been on sick leave since March 1940. Due to this, the retired Kommandørkaptein (Commander Senior Grade) , who lived in nearby Drøbak, had been assigned as temporary commander for the battery. As an unidentified flotilla started forcing its way past the outer fortifications in the south of the Oslofjord, late at night on 8 April, Oberst Eriksen called Anderssen in and had him come down to the fortress. Kommandørkaptein Anderssen donned his old uniform and was transported by boat over the fjord to the torpedo battery. Anderssen would show himself worthy of the important task of leading the fortress' most lethal weapon system; having first served at the torpedo battery in 1909, he knew the ageing weapons intimately. When Anderssen had been called back into duty a month previously, he had been a pensioner for 13 years, having originally retired from his post as commander of the torpedo battery in 1927. The battery had three torpedo tunnels which could fire six torpedoes without reloading and a total of nine torpedoes were stored and ready for use.
Battle
Main Battery rounds
While the main combat station for the Main Battery and the commander of Oscarsborg fortress was on the island Håøya north-west of South Kaholmen (), due to the special circumstances in 1940, Oberst Eriksen took position in the backup station on the eastern flank of the Main Battery at South Kaholmen.
At 04:21 on 9 April, Eriksen gave the Main Battery guns the order to fire at the lead ship of the unknown flotilla forcing its way towards Oslo. Upon giving the command, Eriksen was questioned. He responded with his now famous response; "Either I will be decorated or I will be court martialled. Fire!" Two rounds from the Krupp guns Moses and Aron engaged the German cruiser at range. The two Norwegian guns had been loaded with live, high-explosive shells; firing them "in anger" was a violation of the pre-war Norwegian rules of engagement which dictated warning shots be fired first, as had been the case at the Rauøy Fortress and the Bolærne Fortress further down the fjord. Colonel Eriksen later explained his decision by alluding to the fact that the German naval force already had forced their way past the fortresses further South in the Oslo Fjord and had received both warning shots and live rounds from these more outlying coastal fortifications. As the vessels had continued up the fjord toward the capital, Eriksen was of the opinion that he had the right to consider them enemy warships and to engage them as such.