Donald Trump just imposed a 25 percent tariff on virtually all goods produced by America’s two largest trading partners — Canada and Mexico. He simultaneously established a 20 percent across-the-board tariff on Chinese goods.

As a result, America’s average tariff level is now higher than at any time since the 1940s.

Meanwhile, China and Canada immediately retaliated against Trump’s duties, with the former imposing a 15 percent tariff on American agricultural products and the latter putting a 25 percent tariff on $30 billion of US goods. Mexico has vowed to mount retaliatory tariffs of its own.

This trade war could have far-reaching consequences. Trump’s tariffs have already triggered a stock market sell-off and cooling of manufacturing activity. And economists have estimated that the trade policy will cost the typical US household more than $1,200 a year, as the prices of myriad goods rise.

All this raises the question: Why has the US president chosen to upend trade relations on the North American continent? The stakes of this question are high, since it could determine how long Trump’s massive tariffs remain in effect. Unfortunately, the president himself does not seem to know the answer.

In recent weeks, Trump has provided five different — and contradictory — justifications for his tariffs on Mexico and Canada, none of which make much sense:

1) Trump’s tariffs on Canada and Mexico are meant to secure America’s borders

Officially, the North American tariffs are intended to fight “a drug war,” not a trade war. Trump claims that our neighbors to the north and south have been abetting the transfer of fentanyl and undocumented immigrants into the United States — and that this constitutes a “national emergency.” By imposing steep tariffs on Canada and Mexico, Trump ostensibly aims to coerce the two nations into cutting off the flow of drugs and migrants.

But this policy is irrational. Canada plays virtually no role in the trafficking of fentanyl. And the Mexican government has already ramped up enforcement against........

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