There is no need to reopen the Obama Justice Department’s painstaking investigations of nearly two dozen police departments accused of widespread abuse, or the court-enforced agreements the department reached requiring cities like Cleveland, Seattle and Ferguson, Mo., to enact reforms. When Attorney General Jeff Sessions suggested that he might back away from those agreements, he was playing to police officers who have bristled at calls to root out racist and unconstitutional practices that have been well documented by the Justice Department.

On Friday, Mr. Sessions failed to derail one such agreement, known as a consent decree, when a federal judge ignored his request for a delay and signed an agreement on sweeping police reforms that Baltimore had negotiated in the waning days of the Obama administration. After Freddie Gray, a 25-year-old African-American, died of a broken spine in police custody in 2015, a Justice Department investigation uncovered a pattern of policing in African-American neighborhoods so brutal that people were fearful of cooperating with officers.

Mr. Sessions has argued that consent decrees usurp state and local authority. But the nation’s most broken police departments have proved incapable of reforming themselves unless the federal government intervenes.

Congress came to that conclusion in 1994, when it gave the Justice Department authority to restructure troubled departments after Los Angeles erupted in riots following the acquittal of officers who were videotaped brutally beating a black motorist, Rodney King. The Justice Department became particularly aggressive with this authority under the Obama administration, which opened 23 investigations. The inquiries into Ferguson, Cleveland, Chicago and Baltimore revealed particularly barbaric and racist policing practices.