During the first couple of nights of violence, Minneapolis barely managed to arrest anyone.

For his part, the governor of Minnesota, Tim Walz, hesitated to mobilize the National Guard lest he seem “oppressive,” apparently unaware that his target audience wasn’t a social-justice seminar at Oberlin College but provocateurs and nihilists who were going to take every inch they were given and make it a mile of broken glass and looted goods.

President Donald Trump has been a steady fount of inflammatory and crude statements over the last week, but when he blasted the “total lack of leadership” in Minneapolis, he wasn’t wrong.

All that state and municipal leaders need to know about controlling riots is obvious from a cursory review of the history.

Consider the worst disturbance in recent times, the L.A. riots. They began when about two dozen cops retreated before an angry crowd after the Rodney King verdicts, some of them literally running away. The mob descended on the intersection of Florence and Normandie and began beating Anglo and Latino motorists, completely unmolested by the authorities for hours. Some police reported being ordered to leave the area—and then being ordered not to return. A couple of squad cars drove through the intersection without stopping.

The rest is history: days of violence, more than 60 people killed and 2,000 injured, and in excess $1 billion in property damage. By the end, thousands of federal troops were in the city.

Back in 1970 in his classic book on domestic unrest, The Riot Makers, Eugene Methvin identified police absence or pullback as the accelerant on riots. It was a huge factor in the Watts riots in 1965. The same dynamic held in a Philadelphia riot the year before (an officer there expressed exactly the same sentiment as a retreating LAPD officer in 1992, “The hell with it. Let them do what they want”). In Detroit in 1967, cops retreated, and the authorities underestimated the forces they needed as a riot devastated the city.

It is simply not true that rioters will be quickly sated if they are allowed to break and burn things freely. Disorder feeds on itself. Looting one store, overturning one police car is never enough.

There is no alternative to imposing curfews, zealously enforcing them, arresting violators, and calling out the National Guard if there’s not enough police manpower for the job. This doesn’t escalate the violence, it stops it.

Over the weekend, Minneapolis finally got more serious about policing itself and saw a drastic diminishment of destruction. Anyone who doesn’t want American cities to burn should take note.