Martin Luther King, Jr. has been quoted as saying:
If your opponent has a conscience, then follow Gandhi. But if you enemy has no conscience, like Hitler, then follow Bonhoeffer.
I made use of this quote here. I have been unable to find the quote in a major reference work, or a cursory glance of King's original writings. The most authoritative source that I can find is "The Amazing Grace of Revolution." by Laurie J. Taylor in the Chicago Theological Seminary Register. v.91, n.1 (2001), p.30.