HERMOSILLO, Mexico—The moment a pistol was pressed against the side of cattle rancher Aaron Cameron’s head two years ago, he knew that the business he and his family had worked so hard for was about to be taken away.

Dozens of cartel gunmen drove onto his property in armored vehicles, some mounted with .50-caliber machine guns. They took over the site for more than a year, until state police officers were able to clear them out. Other residents in the area were also targeted.