On July 7, 2016, just days after the deaths of two black men, Philando Castile and Alton Sterling, at the hands of police officers provoked national outrage, a 25-year-old African-­American Army veteran named Micah Xavier Johnson trained his semiautomatic rifle on the police patrolling at a Black Lives Matter protest in Dallas. The officers were not carrying shields or wearing riot gear. By the end of the night, four police officers and one Dallas Area Rapid Transit officer were dead, and nine other officers and two civilians were wounded. Johnson holed up inside a community college, where he became the first person in the United States to be killed by a bomb-­carrying police robot.

The violence was all the more devastating because over the previous four years, David Brown, then the Dallas chief of police, had revamped the department’s lethal-­force policies. Brown had hard-won knowledge about police shootings — in 2010, mere months after he became chief, his mentally ill son was shot and killed by a policeman in the Dallas suburb of Lancaster after he killed an officer and a civilian. Brown’s officers were now required to take refresher courses on de-­escalating conflict, and the department regularly shared all use-of-force data with the public. After these changes were made, the number of officer-­involved shootings dropped from 23 in 2012 to 13 in 2016. (This year, there have been seven instances, two of them fatal.) Excessive-­force complaints against the department fell 74 percent over the same time period.