Imagine we are sitting at a table - a roadside café in Frankfurt, Germany.
I tell you, "I had a nightmare. We were here, but it was 1946 instead. The Americans and Russians have bombed all of Europe into the dust, because this new guy running Germany invaded Poland, Russia, France... he won approval to call the Jews inhuman, and killed millions of them in gas chambers, with machine guns, diseases and starvation. Tens of millions more died in Europe trapped between immense armies - the Americans put thousands of bombers in the sky at one time."
Quietly, you talk to the waiter when he next approaches, and shortly the ambulance arrives. Smiling men in white usher me off to the hospital. Clearly I am mentally ill. No one could do what I suggested that Führer was planning; Germany is starting a new age of recovery.
Every generation is living in the most modern society it has ever known. None of them actually ask for the purges taking millions of people to early graves.
But it happens, largely because no one can believe it, and so no one acts early to prevent it.
Take the lesson.