"Terror itself was not the end goal of those regimes," Gessen writes of Nazi Germany and Stalin's USSR, "but nothing that followed would have been possible without it."

For this was the secret about the secret police that became clear when the K.G.B. archives were opened (briefly) in the 1990s: They were ruled by quotas. Local squadrons had to arrest a certain number of citizens so they could be designated enemies of the people. That the officers often swept up groups of colleagues, friends and family members was probably a matter of convenience more than anything else. Fundamentally, the terror was random. That is, in fact, how state terror works.

"The randomness is the difference between a regime based on terror and a regime that is plainly repressive," Gessen explains. "Even in brutally repressive regimes, including those of the Soviet colonies in Eastern Europe, one knew where the boundaries of acceptable behavior lay."

Open protest would get one arrested; kitchen conversation would not. Writing subversive essays or novels or editing underground journals would get one arrested; reading these banned works and quietly passing them on to friends probably would not. A regime based on terror, on the other hand, deploys violence precisely to reinforce the message that anyone can be subjected to it.

When we think of the terror regimes of the past, it is tempting to superimpose a logical narrative on them, as though totalitarian leaders had an extermination to-do list and worked their way through it methodically. This, I think, is how most people understand Martin Niemller's classic poem "First They Came." In reality, though, the people living under those regimes never knew which group of people would be designated an enemy of the state next.