Half-NAFTA Not The Trump White House believes it has Canada cornered, but deft lobbying of the U.S. Congress by Ottawa makes it unlikely that a solely U.S.-Mexico trade agreement would be approved on Capitol Hill. Meanwhile, Mr. Trump and his counterpart Justin Trudeau have carved out positions on dairy that defy a ready solution.

Markets will not like the administration’s solution, which is to announce withdrawal from NAFTA. Candidate Trump said that he would kill NAFTA if he could not renegotiate it to his liking. While even ending the agreement has its own massive challenges, his goal is to force Canada (and the Congress) to bend to his will or else all will suffer from his destabilizing the North American market. The president...