Following the Nazi invasion of Norway, the daily newspaper "Arbeiderbladet" was closed down during 1940 by Nazi officials. Bratteli subsequently participated in the Norwegian resistance movement. He was arrested by agents of Nazi Germany in 1942, and was a Nacht und Nebel prisoner of various German concentration camps; including Natzweiler-Struthof, from 1943 to 1945. He was liberated from Vaihingen an der Enz concentration camp on 5 April 1945, by the Swedish Red Cross White Buses along with fifteen other Norwegians who had survived.