WASHINGTON, D.C. — The U.S. administration and its congressional allies are continuing to make the case that America’s northern border is facing a rising national security threat. Two weeks ago, Homeland Security Secretary Markwayne Mullin said during a CSIS panel discussion in Washington that pressure on Mexican cartels at the southern border was pushing criminal operations and fentanyl trafficking northward toward the Canadian border. Drug researchers notably questioned the claim. But on Tuesday, a Congressional Subcommittee on Border Security and Enforcement and the Subcommittee...