Could non-violence have been used to stop the Holocaust? Sounds crazy, doesn't it.
However there is the interesting story called the Rosenstrasse protest. Allow me to summarize the story. It happened in 1943. Nazis rounded up the last of the Jews in Berlin with the intent of shipping them to concentration camps. However many of them were married to German woman (who were not rounded up). These women showed up at the prison and peacefully protested for about a week (about 6,000 in total). What happened? The Jewish men were released - in fact 25 men were even brought back from Auschwitz.
Perhaps non-violence could have stopped the Holocaust.