Donald Trump will break most of his campaign promises. Which promises will he keep?

The answer, I suspect, has more to do with psychology than it does with strategy. Mr. Trump is much more enthusiastic about punishing people than he is about helping them. He may have promised not to cut Social Security and Medicare, or take health insurance away from the tens of millions who gained coverage under Obamacare, but in practice he seems perfectly willing to satisfy his party by destroying the safety net.

On the other hand, he appears serious about his eagerness to reverse America’s 80-year-long commitment to expanding world trade. On Thursday the White House said it was considering a 20 percent tariff on all imports from Mexico; doing so wouldn’t just pull the U.S. out of NAFTA, it would violate all our trading agreements.

Why does he want this? Because he sees international trade the way he sees everything else: as a struggle for dominance, in which you only win at somebody else’s expense.

His Inaugural Address made that perfectly clear: “For many decades we’ve enriched foreign industry at the expense of American industry.” And he sees punitive tariffs as a way to stop foreigners from selling us stuff, and thereby revive the “rusted-out factories scattered like tombstones across the landscape.”