On Tyranny by Timothy Snyder

If you carry a weapon in public service, may God bless you and keep you. But know that evils of the past involved policemen and soldiers finding themselves, one day, doing irregular things. Be ready to say no.

AUTHORITARIAN REGIMES USUALLY include a special riot police force whose task is to disperse citizens who seek to protest, and a secret state police force whose assignments include the murder of dissenters or others designated as enemies. And indeed we find forces of the latter kind deeply involved in the great atrocities of the twentieth century, such as the Great Terror in the Soviet Union of 1937–38 and the Holocaust of European Jews perpetrated by Nazi Germany in 1941–45. Yet we make a great mistake if we imagine that the Soviet NKVD or the Nazi SS acted without support. Without the assistance of regular police forces, and sometimes regular soldiers, they could not have killed on such a large scale.