At the 1968 Democratic National Convention, law enforcement in Chicago used force against protesters; at the 2016 Republican National Convention, Cleveland police say they plan to use peaceful crowd control. PHOTOGRAPH COURTESY BETTMANN / GETTY

It would be hard to imagine a more nerve-wracking set of crowd-management circumstances than the ones coming together next week in Cleveland, where tens of thousands of people are expected to protest outside the Republican Convention. The groups signed up are an All-American trail mix, ranging from Black Lives Matter to Code Pink, to Bikers for Trump. The Republican nominee is a man who thrives on fomenting anger in the crowds who follow him. And lately some anti-Trump activists have responded in kind, roughing up Trump supporters at rallies in San Jose and Albuquerque. The Convention starts on the heels of the shooting of multiple police officers in Baton Rouge; four days after the horrific terrorist attack on a gathering of people watching Bastille Day fireworks in Nice; less than two weeks after the ambush killings of five police officers in Dallas, by a lone sniper taking advantage of a protest march; and more than two years into a civil-rights movement that has made the police shootings of unarmed black people impossible, at last, to ignore.

Cleveland’s police department, the primary entity in charge of maintaining public safety while allowing demonstrators to exercise their constitutional rights, has been operating since June, 2015, under a consent decree with the Department of Justice. A D.O.J. investigation had faulted the department for, among other things, excessive use of lethal force, including a pattern of officers unnecessarily shooting people and hitting them on the heads with guns, as well as endangering bystanders. (One of the events that led to the investigation was a high-speed chase in 2012 that involved a hundred Cleveland police officers in sixty-two police vehicles tracking a single car, at which they fired a hundred and thirty-seven times. The two unarmed people in the car, who were ultimately killed, were both black.) Because Ohio is an open-carry state, people in the protest zone outside the Quicken Loans Arena, where the Convention will be held, will be allowed to have guns on them (though they will be prohibited from bringing a long list of other items, including water guns, canned goods, and, for some reason, tennis balls).

Some of the tension in the air is explicitly racial. As Bill Daher of Bikers for Trump told Politico last week, “Well, hey, when the shooting starts, if the black people start dropping, then some black person’s hiding behind me. If the white people start dropping, then I’m going to hide behind a black person. So, I don’t know what’s going to result.”

At least it’s predicted to be seasonably warm and not so humid, since a good deal of research indicates that, as one recent study noted, hot temperatures really do “increase aggression by directly increasing feelings of hostility and indirectly increasing aggressive thoughts.”

Still, if law enforcement in Cleveland can manage to keep it together despite all this, they have a lot of new thinking to draw on in the management of big, potentially unruly crowds. In 1968, when untrammelled street protest and high-stakes electoral politics collided at the Democratic Convention, in Chicago, the prevailing mode of crowd control was the crushing show of force. It was an approach that derived from the origins of crowd theory, in nineteenth-century France, which had a history peppered with urban uprisings. This theory, developed by sociologists such as Gustave Le Bon, saw the crowd as a kind of monstrous living organism, in which individual identities were inexorably subsumed, subject to widespread contagion, from a single germ, or bad actor. That meant that any misbehavior had to be stamped out before the contagion spread.

Not that the cops in Chicago needed much in the way of sociological theory when it came to beating down some hippie addressing them as swine. In his memorable first-person account “No One was Killed: The Democratic National Convention, August 1968,” the journalist John Schultz describes one typical scene this way:

The cops came out of that bus as if they were shot from a gun, howling and running as hard as they could after screaming kids who were running as hard as they could to get away. The [National] Guardsmen simply stood on the edge of the park across the street and watched. The cops were beating everyone in reach with their clubs, jamming them up against the wall of the building, against the iron fence of the Georgian Court, ramming them in the groin, and if a kid were caught in a no-exit situation between cars he was beaten senseless. . . . If there was one thing that characterized the Chicago cops, it was energy, tremendous energy, a high pitch of personal energy, and a nonchalance that could change on the instant into white-faced fury.

In the years since, especially in Europe, sociologists developed a new theory of crowd behavior called the Elaborated Social Identity Model. The idea was that demonstrators in crowds behaved (surprise!) in varied ways—some might try and stop looters or stone-hurlers in their midst; some might join them—but that, in general, an aggressive police response, especially if it thwarted the original purpose of the protest, had the effect of uniting what was otherwise a disparate mass of people. As Eric Jaffe, writing for the Web site Co.design, explains: