The crackdown on the Occupy movement now has its two signature moments. Both involve pepper spray. On September 25th, a New York City Police Department inspector sprayed a group of women who were penned in and defenseless during a protest in lower Manhattan. Then, on November 18th, police officers at the University of California, Davis, sprayed a group of protesters who were seated, and unresisting, during a campus protest.

Officers have been suspended in both cases, and investigations are under way. But these incidents, plus others around the country, raise the question: What’s going on here? In my view, it’s something more than just a spate of over-aggressive policing. My bet is that we can point to two larger trends that may be responsible: “broken windows” and 9/11.

One of the unambiguously positive changes in American life in recent years has been the dramatic reduction in crime. New York has seen the most extraordinary changes; for example, there were five hundred and thirty-two homicides in 2010, down from twenty-two hundred and forty-five in 1990. But over the past two decades there have been declines in violent crime almost everywhere. There are spirited debates about the causes of this happy development, but “broken windows” almost certainly deserves some of the credit. Initially based on a 1982 article by James Q. Wilson and George L. Kelling, the broken-windows theory of police enforcement holds, in rough terms, that aggressive pursuit of minor crimes will lead to a reduction of major crimes. (In 1997, David Remnick wrote a profile of Jack Maple, the local cop who was most responsible for introducing these ideas to the N.Y.P.D.) The theory was straightforward, and believable. If you make a concerted effort to arrest turnstile-jumpers, you’ll find that you’re catching serious bad guys in the process. A graffiti artist is sometimes a true perp.

A post-9/11 shift in law enforcement led to similar changes in police activity. The traditional model had been to arrest people after they committed crimes. But the magnitude of the risks connected with terrorism raised questions about the efficacy of this approach. As I learned shortly after 9/11, when I profiled Michael Chertoff, then the Assistant Attorney General in charge of the criminal division (and later the Secretary of Homeland Security), and John Ashcroft, the Attorney General, they both felt that law enforcement had to move to a preventative model. The goal was to use any tool at their disposal—immigration detentions, material-witness warrants—to stop crimes before they took place. This was the motivation behind the USA Patriot Act, which passed Congress nearly unanimously in the aftermath of 9/11. They wanted to give law enforcement every possible scrap of information so that the authorities could intercept and prevent terrorist acts. Prosecution after the fact would be a catastrophic failure.

So, in a broad sense, the broken-windows theory and the 9/11 transformation pushed police in the same direction: to act quickly. In this way, it appeared, they could limit risks. The underlying theories for both developments held that it was better to arrest violators sooner rather than later, and that the risk of acting quickly was less than the risk of being too slow. Crime continued to decline, and there were no major terrorist attacks, so it appeared that that changes in tactics have worked.

But as the Occupy pepper-spraying shows, there is a cost to police aggressiveness as well. We do not yet know what was going through the heads of the police officers who doused defenseless protesters with these toxic chemicals. But it is safe to assume that they thought the risks of waiting were outweighed by the risks of taking the fight to the Occupy group. After all, that’s what has been working lately, around the country.

Other police virtues seem to have been lost in the process, at least when it comes to the Occupy movement. Neither the broken-windows approach, nor the counter-terrorism model, puts much of a premium on patience. But often that’s what cops need. Tensions cool; anger burns out; the weather turns cold. Time will break up a rally, or a sit-in, just as surely as pepper spray will, but time takes a lot longer. Waiting can be frustrating, and boring, but it’s often very effective and the risk of violence is low. If there is one message of the pepper-spray excesses over the past couple of months, it’s that time is sometimes worth the wait.

Photograph by Justin Sullivan/Getty Images.