In 1940 Norway was invaded and occupied by Nazi Germany. Arntzen worked to spread resistance to the occupation throughout his profession. In 1942 he was recruited by Haakon Sund as a police organizer within the Norwegian resistance movement. Arntzen became a member of Milorg's military council and Hjemmefrontens Ledelse. From May 1944 to September 1944 he was incarcerated at Bredtveit prison. The authorities not being aware of his association with Milorg and Hjemmefrontens Ledelse, he was then released from prison, only to continue his resistance work. He helped plan the situation which would follow the German defeat in World War II.