If lawyers had followed the norm of no execution without trial, if doctors had accepted the rule of no surgery without consent, if businessmen had endorsed the prohibition of slavery, if bureaucrats had refused to handle paperwork involving murder, then the Nazi regime would have been much harder pressed to carry out the atrocities by which we remember it.

Dead human beings provided retrospective arguments for the rectitude of policy. Hitler and Stalin thus shared a certain politics of tyranny: they brought about catastrophes, blamed the enemy of their choice, and then used the death of millions to make the case that their policies were necessary or desirable. Each of them had a transformative utopia, a group to be blamed when its realization proved impossible, and then a policy of mass murder that could be proclaimed as a kind of ersatz victory.

History does not repeat, but it does instruct.