A phrase which has been said by the locals in just about every situation involving the worst atrocities in human history; “It can’t happen here”, -or- “we are more civilized/advanced/humane than that”. After the atrocity those phrases are preceded by “I thought...”
The Constitution. Learn it, or regret not learning it.
That would make a good cartoon — two panels side by side, one panel with a person reading a scroll, holding it in his hands. The other panel, the same person, with hands in pretty much the same position but holding onto prison bars instead.