OTTAWA — This year on Canada Day, Prime Minister Justin Trudeau will visit a former Heinz ketchup plant in Ontario and, later Sunday, meet steelworkers in Saskatchewan.

It’s a schedule rich with the symbolism of the moment: Sunday is also the day Canada retaliated against the Trump administration’s tariffs on Canadian steel and aluminum with import duties on $12.6 billion of American products, from ballpoint pens to industrial pipes. The workers Mr. Trudeau is visiting will be in the middle of this fast-escalating trade war.

The White House’s use of a national security argument to justify the duties against a close ally, along with President Trump’s repeated belittling of both Mr. Trudeau and his trade policies, has offended and angered Canadians. On social media, they are calling for boycotts of American products and encouraging one another to look elsewhere for vacation destinations. Mr. Trudeau’s decision to retaliate won a rare endorsement from all three of Canada’s major political parties.

“We’re living in a brand new world,” said Debra Steger, a law professor at the University of Ottawa who is a former senior trade negotiator for Canada and a onetime official at the World Trade Organization. “It has been pretty messy, and it’s not going to get better soon. We really are in a very difficult time.”