It happened, therefore it can happen again: this is the core of what we have to say. It can happen, and it can happen everywhere. - Primo Levi, 1986 The Drowned and the Saved
Below is a video from the United States Holocaust Memorial Museum on the path that led to the genocide in Germany.
Unfortunately the late Nat Hentoff pointed to another holocaust that we could have stopped in Rwanda in 1994, but the world failed the people of that nation.
Today we maybe witnessing another in Syria targeting religious minorities, including Christians.