Illegal border crossing: How U.S. guns wind up in Canada and Mexico

America’s neighbors have long lived with the cross-border creep of U.S. gun culture – sometimes with outrage, often with a sense of resignation.

But the mass shootings in El Paso, Texas, and Dayton, Ohio, this month have emboldened foreign stances against lax U.S. gun laws and shifted conversations about what that means for crime and safety north and south of the U.S. border.

The shooting inside a Walmart in El Paso that claimed 22 lives in a matter of minutes was far more than just an American tragedy. More than...