Canada accounted for more than half of American imports of aluminum in 2016, followed distantly by Russia and the United Arab Emirates. Canada also made up the largest share of American steel imports in 2016 — 17 percent — followed by Brazil, South Korea, Mexico and Turkey. China did not rank among the top 10 suppliers of either metal.

“From a defense perspective, it makes no sense to limit imports from Canada in the future,” said Alf Barrios, the chief executive of Rio Tinto Aluminum, which exports aluminum from Canada to the United States. “Canada is a longstanding, reliable supply for any needs the United States might have.”

Canada’s inclusion in the Commerce Department recommendations was unexpected, in part because companies and workers have been relatively united in calling for the country to be exempted. The operations of many metal makers — as well as the United Steelworkers, their largest union — stretch across the border. Under law, Canada is included as part of the United States defense industrial base, and the growth of its aluminum industry dates back to supporting the United States during World War II.

Kathleen Wynne, the premier of Ontario, said the recent level of animosity and trade tensions with the United States had surprised Canadians. “We see ourselves as close friends,” she said. “It’s unexpected that we would stand to be damaged by the United States, and vice versa.”

Ken Neumann, the national director of United Steelworkers, struck a similar note. “There is no justification to include Canada with countries that systematically violate trade laws and engage in the dumping of illegally subsidized aluminum and steel,” he said after the release of the Commerce Department report.

Canada is one of several allies that could be affected. Britain, Australia, Europe and Japan would also be included under the kind of blanket tariff or quota recently floated by the Commerce Department.