On Friday, the United States and Mexico agreed to a new joint strategy to stem the flow of weapons that helps fuel cartel violence. Named “Mission Firewall,” the plan seeks to disrupt the illicit flow of arms across the border by expanding the use of eTrace — a digital system that allows authorities to trace the origins of firearms recovered at crime scenes — in all 32 Mexican states and by introducing ballistic imaging technology nationwide. It also aims to strengthen intelligence-sharing and expand joint investigations. For its part, Washington has pledged to increase border inspections and coordinate more closely with Mexican authorities to try to halt the relentless southbound stream of weapons.

All of this is good news. Combating arms trafficking is an indispensable step toward weakening Mexico’s criminal organizations. Yet behind the official enthusiasm lies a glaring hypocrisy on the U.S. side of the equation. No matter how often officials........

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