Efforts by Prime Minister Mark Carney to signal pragmatism and bonhomie in his meetings with Donald Trump have been undermined by other messages that rile Mr. Trump on issues far from trade.Mark Schiefelbein/The Associated Press

Fen Osler Hampson is a professor of international affairs at Carleton University and the co-chair of the Expert Group on Canada-U.S. Relations.

Henry VIII divorced or executed his wives not because they failed in their duties, but because they could not satisfy his constant quest for personal validation and desire for a male heir. Canada is dealing with a sovereign with a similar outsized ego in the Canada-United States-Mexico Agreement trade negotiations.

U.S. businesses and consumers want the marriage to last. However, the fate of the agreement turns less on shared interests than on the fickle moods of Donald Trump – the king – in Washington.

On the fundamentals, the case for renewal is overwhelming. Cross‑border supply chains now form a single, seamless production line. Industries on both sides of the border have said in overwhelming numbers that USMCA works well. Consumers........

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