"Those who forget history are doomed to repeat it", or something similar. And since we began teaching about the Holocaust, there has not been a single occurrence of genocide. Well, apart from Rwanda. And Bosnia. Oh and Darfur. There's a short article
here on the subject.
"Those who forget history are doomed to repeat it - and not know it. Those who remember history are doomed to repeat it - and know full well but hey, we are human"