The new breed of district attorney threatens a deeply entrenched system, with tentacles in multiple agencies and the backing of many of the police officers, judges, rank-and-file prosecutors and probation and parole officers who determine what justice, or at least law enforcement, actually means day to day. Some of those insiders stiffly resisted the calls for change. When Larry Krasner took over the district attorney’s office in Philadelphia in 2017, more than 100 lawyers and employees walked out the door; Krasner also dismissed 31 others. (Gasc?n, by contrast, couldn’t clean house: In Los Angeles, prosecutors have the rare benefit of civil-service protections and a union.)