After 1939 most active Jehovah's Witnesses were incarcerated in prisons or concentration camps. Some had fled Germany. In the camps, all prisoners wore markings of various shapes and colors so that guards and camp officers could identify them by category. Witnesses were marked by purple triangular patches. Even in the camps, they continued to meet, pray, and make converts. In Buchenwald concentration camp, they set up an underground printing press and distributed religious tracts.
Conditions in Nazi camps were generally harsh for all inmates, many of whom died from hunger, disease, exhaustion, exposure to the cold, and brutal treatment. But, as psychoanalyst Bruno Bettelheim and others have noted, Witnesses were uniquely sustained in the camps by the support they gave each other and by their belief that their suffering was part of their work for God. Individual Witnesses astounded their guards with their refusal to conform to military-type routines like roll call or to roll bandages for soldiers at the front. At the same time, Witnesses were considered unusually trustworthy because they refused to escape from camps or physically resist their guards. For this reason, Witnesses were often used as domestic servants by Nazi camp officers and guards.
According to Rudolf Höss, Commandant of Auschwitz, SS Chief Heinrich Himmler often used the "fanatical faith" of Jehovah's Witnesses as an example to his own SS troops. In his view, SS men had to have the same "unshakable faith" in the National Socialist ideal and in Adolf Hitler that the Witnesses had in Jehovah. Only when all SS men believed as fanatically in their own philosophy would Adolf Hitler's state be permanently secure.
In the Nazi years, about 10,000 Witnesses, most of them of German nationality, were imprisoned in concentration camps. After 1939, small numbers of Witnesses from Austria, Belgium, Czechoslovakia, the Netherlands, Norway, and Poland (some of them refugees from Germany) were arrested and deported to Dachau, Bergen-Belsen, Buchenwald, Sachsenhausen, Ravensbrück, Auschwitz, Mauthausen, and other concentration camps. An estimated 2,500 to 5,000 Witnesses died in the camps or prisons. More than 200 men were tried by the German War Court and executed for refusing military service.
During the liberation of the camps, Jehovah's Witnesses continued their work, moving among the survivors, making converts.