Gang violence in Chicago is bad and getting worse. In 2016, 762 people were murdered and more than 4,300 people were shot in Barack Obama’s hometown. The gunplay hasn’t let up in 2017. Although Chicago’s violence has primarily been concentrated on the city’s South and West sides, the downtown area hasn’t been spared.

Mr. Obama, a South Side homeowner, should make reducing Chicago’s violence his first postpresidency project. He and Mayor Rahm Emanuel should form a citywide task force to find a way to stop the killing and start...