After two years of nationwide protests over police treatment of blacks and other minorities, relations between citizens and police remain troubled, even as homicide rates are rising in most of the nation’s largest cities. But it is unclear whether the recent surge in urban violence means the nation’s decades-long decline in overall crime is ending. Justice Department interventions in some cities aim to overhaul departments where officers have exhibited a pattern of using excessive force. And in an era of cellphone and police body-camera videos recording interactions between police and minorities, officers disagree over whether to de-emphasize the use of force in potentially lethal confrontations. Some experts blame a falloff in preventive enforcement for the rise in crime, but others say it is not clear that a decreased enforcement trend is underway.