1.) what facts about the holocaust does your reader need to have in order to understand the event’s importance to new citizens?
2.) why is the holocaust important for new citizens to understand?
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An enormous question.
The Holocaust, which seems to have been the result of German Chancellor's "final solution" to the "Jewish Problem". Kill them all, by the cheapest means possible. And, to do that, the chancellor had to "get his supporters to buy his message". That was done by nationwide indoctrination, anything from banning "Jew physics" (a blunder of a ban, because Jews, Professors Einstein and Lisa Meitner to name but two associated with the Nobel prize in physics, played important roles in developing nuclear weapons), through to indoctination of the dreaded "Hitler youth", via "krystalnacht" and the mass transportation of as many Jews as could be tranported to death camps "hidden" in eastern europe near the USSR border.
And this is only part of the start to the answer.
It may well be impossible to do this subject justice anywhere ... 

No because there were alot of people