Why a Swedish city with a violent crime problem looked to US for solutions

Rebeca Persson remembers clearly the afternoon in October 2018 when she and other Malmö residents first met with 10 young men – many from rival street gangs, and all on parole – in the city’s soccer stadium.

The social worker sat onstage next to the other speakers as the gang members filed into the large hall. An audience of 40 people stood to greet them: neighbors, sports coaches, owners of corner shops, leaders in local mosques, and even relatives of the young men.

Neither Ms. Persson nor the men knew...