If you want to vanquish your enemies, you would want to gather them in one place, concentrate them. And so it has been through the ages, by Stalin, Mao, every despot, everytime a sizable minority was being “culled.” The concentration camps for the Jews were among the most extreme, but they were not only for the Jews, other dissidents and partivularly Christians were in these camps:
The Holocaust concentrated on the Jews, but Catholics and especially their priest leaders were targeted as well. In Dachau alone, no fewer than 2,771 priests were imprisoned, of whom at least 1,000 died from hunger, disease or illtreatment. Acts of brutality, torture and murder were commonplace in these camps, not just for Jews and Christians, but other dissidents, especially Christians. The majority of the priests interned in Dachau were of Polish origin; however, apart from German nationals, there were large numbers of French, Czechs, and Austrians. Dachau was host to priests from all over Nazi occupied Europe. Seminarians from these same countries were drafted in as part of forced labour gangs in Germany.
No less than 4,000 priests were put to death during these years, either as 'political saboteurs', or, after incarceration in concentration camps, by hanging, starvation, mishandling, lack of medical aid, or as victims of medical experiments including euthanasia. It is a story of courageous and heroic resistance against the overwhelming power of a police state. 
1. cf Lewy, G., The Catholic Church and Nazi Germany, London 1964, pp.170-172
2. cf Homiletic and Pastoral Review, February 1983, p.48-49
3. cf J.C. Conway, The Nazi Persecution of the Churches 1933-1945, London 1968, pp.298-299, and note 24, p. 447