Early resistance activities Sønsteby was working as an accountant when the
Germans occupied Norway in 1940. Norway's regular armed forces surrendered on 10 June 1940, after two months of fighting, and the country was subsequently occupied by the Germans. He quickly joined the
Norwegian Resistance forces in Østlandet. He fought in Philip Hansteen's ski company. He was also involved in the underground press. After
D-Day, Sønsteby concentrated largely on bombing Norwegian railways, thereby keeping German reinforcements from being moved back to the front line. His team also sank the
German transport ship Donau outside
Drøbak in 1945.
Disguise and hiding Operating in occupied territory, and being high on the
Gestapo list of wanted men, Sønsteby became a master of disguise. He operated under 30 to 40 different names and identities, and the Germans did not acquire his real name until near the end of the war. They were never able to catch him. His obituary in
Aftenposten attributed his ability to elude capture to "resourcefulness, luck, intuition", and "such an entirely ordinary appearance that he was hardly noticed when he rode his bicycle through Oslo's streets." Sønsteby's father was arrested by the Gestapo and held until December 1944.
Assassinations of informants When Sønsteby was 80 years old, he said "Of course wrong decisions were made, also by the Resistance Movement. But one must remember that war was going on. It did happen that we had to kill without being sure that the person concerned was an informant. But the decisions were correct—there and then." ==Post-war activities==