President Donald Trump has pushed into new trade law territory with an emergency sanctions law to justify punishing 25% tariffs on Canadian and Mexican imports and an extra 10% duty on Chinese goods to curb fentanyl and illegal immigration into the U.

Mr. Trump is trying use tariffs to force Mexico to better guard its border with the United States, but Mexican officials say the tariffs will only hurt both countries.

Trump knows the United States has more leverage over its neighbors than they do over his country. But exploiting that position of strength could cause a historic rupture.