MEXICO CITY—A son of the infamous Mexican drug lord Joaquín “El Chapo” Guzmán is captured. Cartel gunmen respond with a vicious attack on soldiers and civilians across a major Mexican city, leaving at least eight people dead and 16 wounded. The government gives in and releases the son, a top figure in the cartel.

One of the most violent and harrowing days in Mexico’s long fight against drug cartels unfolded late Thursday as members of the Sinaloa cartel wreaked havoc across Culiacán, a modern, middle-class city of around 800,000...