In early April, shortly after Markwayne Mullin took over the Department of Homeland Security, he floated an idea on Fox News that wasn’t taken seriously; it sounded, in fact, like a proposal from someone very new on the job: Mullin threatened to cut federal screening of international passengers and cargo at airports in cities with “sanctuary” policies, which limit cooperation with Immigration and Customs Enforcement. Such a move would trigger flight cancellations to airports in New York, Los Angeles, Chicago, and other major cities and force airlines to reroute to other destinations. Mullin’s proposal seemed more like a wild swing than a real plan. The new secretary is pushing forward anyway. Last Wednesday, Mullin convened a small group of airline and travel-industry executives at DHS headquarters in Washington and told them he may reduce Customs and Border Protection staffing at major airports that serve sanctuary jurisdictions. Mullin told the executives the locations could include Portland International Airport, in Oregon; New York City–area airports such as John F. Kennedy International Airport and Newark Liberty International Airport; …