It would make a grave problem worse

Los Angeles

It’s dark outside and difficult to see through the tinted windows. At the wheel is Sergeant Yolanda Valento of the human-trafficking unit of the Operations-South Bureau of the Los Angeles Police Department. She’s in plain clothes, wearing a bulletproof vest, and carrying a 9mm Glock. The police radio warns of a nearby shooting with victims. We make a U-turn on Figueroa Street, the spine of South L.A. and one of the most notorious prostitution tracks in the country.

On the corner of 79th Street two male officers are standing, talking to an 18-year-old girl. She’s wearing a see-through neon dress, …