Ships visiting Southern California’s bustling ports are poised to become the region’s largest source of smog-causing pollutants in coming years, one reason state regulators want to reduce emissions from thousands more of them.

Over and over in their decadeslong war on smog, Southern California regulators have failed to use a powerful tool against ports, warehouses and other freight and logistics hubs that are magnets for air pollution.

The mayors of Los Angeles and Long Beach signed an agreement June 12 directing the nation’s largest port complex to reduce air pollution by moving toward zero-emission trucks and yard equipment.

The Port of Los Angeles has failed to meet pollution-reduction requirements at another shipping terminal, city officials said Feb. 1, their second such admission in recent months.

Shortly after the Port of Los Angeles agreed to require air quality improvements at one of its busiest terminals seven years ago, top port officials privately began rolling back pollution-cutting measures they promised, government records obtained by The Los Angeles Times show.