A federal judge ruled Monday that the stop-and-frisk policies of the New York City Police Department were unconstitutional. That same day, Attorney General Eric Holder announced the Justice Department will pull back from prosecuting low-level drug offenders to avoid triggering harsh mandatory sentences.

Both decisions reflect fundamental changes in U.S. law enforcement practices. The resulting strident opposition to the changes and equally adamant support illuminate the deep disagreements in the nation’s unresolved racial divide.

Holder pointed out that mandatory sentences fell disproportionately on minority communities and had led to grossly overcrowded prisons. Judge Shira Scheindlin ruled that the New York police policy violated the Constitution — police are most often stopping and frisking innocent male minorities.

New York Mayor Michael Bloomberg and Police Commissioner Ray Kelly quickly defended the policy. They both argued that the tactics had greatly reduced violence and crime — and the number of minority crime victims. Supporters of mandatory sentences and stop-and- frisk contend that, most importantly, policy changes would lead to far higher crime levels.

My decade-long experience as a beat officer in New York’s Harlem, the highest crime area in the city during the 1960s, when crime soared, and as police chief of Kansas City, Missouri, and San Jose, California, during the high-crime 1970s and ’80s, convinced me that police tactics and judicial sentencing policies do affect crime rates.