To assist them in deploying this new weaponry, police departments have also sought and received extensive military training and tactical instruction. Originally, only the largest of America's big-city police departments maintained S.W.A.T. teams, and they were called upon only when no other peaceful option was available and a truly military-level response was necessary. Today, virtually every police department in the nation has one or more S.W.A.T. teams, the members of whom are often trained by and with United States special operations commandos. Furthermore, with the safety of their officers in mind, these departments now habitually deploy their S.W.A.T. teams for minor operations such as serving warrants. In short, "special" has quietly become "routine."

The most serious consequence of the rapid militarization of American police forces, however, is the subtle evolution in the mentality of the "men in blue" from "peace officer" to soldier. This development is absolutely critical and represents a fundamental change in the nature of law enforcement. The primary mission of a police officer traditionally has been to "keep the peace." Those whom an officer suspects to have committed a crime are treated as just that—suspects. Police officers are expected, under the rule of law, to protect the civil liberties of all citizens, even the "bad guys." For domestic law enforcement, a suspect in custody remains innocent until proven guilty. Moreover, police officers operate among a largely friendly population and have traditionally been trained to solve problems using a complex legal system; the deployment of lethal violence is an absolute last resort.

Soldiers, by contrast, are trained to identify people they encounter as belonging to one of two groups—the enemy and the non-enemy—and they often reach this decision while surrounded by a population that considers the soldier an occupying force. Once this identification is made, a soldier's mission is stark and simple: kill the enemy, "try" not to kill the non-enemy. Indeed, the Soldier's Creed declares, "I stand ready to deploy, engage, and destroy the enemies of the United States of America in close combat." This is a far cry from the peace officer's creed that expects its adherents "to protect and serve."

The point here is not to suggest that police officers in the field should not take advantage of every tactic or piece of equipment that makes them safer as they carry out their often challenging and strenuous duties. Nor do I mean to suggest that a police officer, once trained in military tactics, will now seek to kill civilians. It is far too easy for Monday-morning quarterbacks to unfairly second-guess the way police officers perform their jobs while they are out on the streets waging what must, at times, feel like a war.

Notwithstanding this concern, however, Americans should remain mindful bringing military-style training to domestic law enforcement has real consequences. When police officers are dressed like soldiers, armed like soldiers, and trained like soldiers, it's not surprising that they are beginning to act like soldiers. And remember: a soldier's main objective is to kill the enemy.

The views expressed by Rizer are entirely his own, and he does not speak for the Department of Justice or the U.S. Army.

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Arthur Rizer is the director of criminal justice and civil liberties at the R Street Institute. He is also an adjunct professor at George Mason University’s Antonin Scalia Law School, a former police officer and federal prosecutor, and a U.S. Army combat veteran.