Norway was
invaded by Germany on 9 April 1940. Coincidentally, Kayser returned to Norway from Finland on this date, crying as he found the capital city
Oslo invaded. He became involved in the
fighting that ensued elsewhere in Norway. On 25 April he fought in the Battle of Skjervet in
Granvin. The Norwegian forces lost, and retreated to
Gudvangen.
Heavy water sabotage In 1942 the British
Operation Freshman, directed against the heavy water production facility at
Vemork, failed miserably. Kayser was one of the first two saboteurs to enter the Vemork facility on 27 February, crawling through a cable shaft, the other being the team leader,
Joachim Rønneberg. They surprised the person who guarded the
heavy water cylinders in the factory—in retrospect the guard was described as "a good Norwegian". and Rønneberg went on to place explosive charges on the heavy water cylinders in the factory. Two more members of the Gunnerside team later joined Kayser and Rønneberg by climbing through a window. During the operation a Norwegian watchman was temporarily detained, and guarded by Kayser, being released 30 seconds before the explosives went off. The saboteurs of Operation Gunnerside escaped unharmed, even though the
German occupying authorities deployed thousands of soldiers to search Hardangervidda for the saboteurs. Kayser made his way to Sweden For his role in the heavy water sabotage Kayser was awarded the
Military Medal by King
George VI of the United Kingdom. Kayser, then a lieutenant, was second-in-command of the base
Bjørn West at
Matre in
Masfjorden Municipality and took part in the 28 April–3 May 1945 fighting after the base was discovered by the Germans. Kayser had arrived in Masfjorden in October 1944 with a fellow Norwegian Independent Company 1 soldier,
second lieutenant Severin Synnes. The two had been transported to Masfjorden from the
UK on the Royal Norwegian Navy
submarine chaser HNoMS Vigra with orders to establish
Bjørn West.
Awards and honours In total, Kayser received fourteen decorations for his wartime efforts. In addition to the Military Medal, he received the
Légion d'honneur and twice the
St. Olav's Medal With Oak Branch. Thus he was referred to as "Western Norway's
Gunnar Sønsteby". ==Post-war life==