On 9 April 1940, Norway was invaded and occupied by Nazi Germany. The Communist Party and "Arbeideren" originally declared themselves neutral. On 16 August 1940, however, the Communist Party was outlawed, and the newspaper "Arbeideren" was shut down by the authorities. Kristiansen was arrested, but was released shortly thereafter. He continued as an underground member of the Communist Party, and is considered as the de facto party chairman during this period.