Chiefs of some of the nation’s biggest police departments say officers in American cities have pulled back and have stopped policing as aggressively as they used to, fearing that they could be the next person in a uniform featured on a career-ending viral video. That was the unifying — and controversial — theory reached Wednesday at a private meeting of more than 100 of the nation’s top law enforcement officers and politicians. With homicide rates soaring inexplicably this year in dozens of U.S. cities, the group convened by new U.S. Attorney General Loretta Lynch concluded with a brief news conference...