TORONTO — It was yet another slap in the face from the bellicose best friend and neighbor to the south.

Of all the countries that could feel the sting of President Trump’s planned tariffs on steel and aluminum imports, Canada, the long-suffering and restrained American trading partner, is at the top of the list because it is the No. 1 exporter of both to the United States.

But as the news settled on Friday, Canadians took another breath and hoped that Prime Minister Justin Trudeau and the army of cheerleaders he has deployed to charm American officials since President Trump was elected would cajole the United States back from the edge of a trade war.

Rather than erupting in rage, Mr. Trudeau — ever conscious of how much his country’s economy relies on the United States — remained calm and firm, counting off the reasons that Mr. Trump’s move defied economic sense.