In 1954 he was convicted by Eidsivating Court of Appeal of treason and espionage in favour of the Soviet Union, and sentenced to eight years in prison.

In January 1937 Sunde led a group of four volunteers he had personally recruited from Oslo to the headquarters of the International Brigades in Albacete, Spain.

[6] From 1938 he was the leader of the Norwegian branch of Ernst Wollweber's secret sabotage organization, whose actions were directed against Fascist-controlled shipping.

[6] According to historian Lars Borgersrud, Sunde's renown as an aggressive and daring saboteur,[8] led other resistance organisations to approach him to carry out particularly delicate missions,[8] in particular assassinations.

[6] After attacking the Oslo and Aker offices of the State Police 21 August 1942, a reward of 50,000 NOK was offered for information that would lead to his capture.

[8] On 30 May 1944 he and 8 other members of "Osvald" are surrounded by German forces, at a former hotel ("Sollia") in Søndre Land Municipality in Oppland county.

*Signe Raassum[10] After the war Sunde wrote the book Menn i mørket (1947) (Men in the Darkness), describing his wartime activities.

The more famous and acknowledged partisan Gunnar Sønsteby unsuccessfully spoke in favour of giving Sunde a war pension, which was given to both the merchant sailors and the military branch of the resistance.