Relations between Canada and the United States are under growing strain after Washington recently threatened to impose a 50 percent tariff on Canadian aircraft, raising concerns among scholars and labor groups about escalating trade tensions and risks to the aerospace sector.

The move highlights a deeper shift in how the US handles trade disputes with Canada, said Robert Bothwell, a professor emeritus of history at the University of Toronto, who described the current state of bilateral relations as at a historic low.

Asked why aircraft have once again become the focus of US trade threats, Bothwell said the issue itself is largely incidental.

The aircraft tariff threat fits a broader pattern in Washington's use of trade measures as a political weapon rather than an economic tool, he said. The issue chosen matters less than the public display of pressure designed to force concessions, he added.

Concerns over the potential effect on Canada's aerospace industry have also been echoed by labor groups.

The country's largest private-sector union, Unifor, said in a statement on Friday that the latest escalation threatens to levy tariffs on and decertify the Bombardier Global series aircraft — and all other aircraft made in Canada.

"History is full of examples of the United States weakening and destabilizing Canadian aerospace manufacturing, from the Avro Arrow to the Bombardier C Series, and now (President Donald) Trump is threatening our world-class Global jet program," said Lana Payne, Unifor's national president.

"Make no mistake, this latest threat is a continued attack on Canada's industrial economy and good union jobs," she said.

The US move would "further undermine Canadian industrial capacity, good jobs and national sovereignty, while dangerously politicizing airworthiness certification processes that exist to protect public safety", Unifor said.

Unifor Quebec Director Daniel Cloutier said "politicizing aircraft certification is deeply troubling".

Bothwell said Canadian Prime Minister Mark Carney's pessimistic view of the bilateral trade outlook reflects growing uncertainty about Washington's reliability as a partner.

"As he has said publicly many times (such as during the World Economic Forum in Davos), relations with the United States are not stable," he said.

Turbulence ahead

Looking ahead to upcoming negotiations on the Canada-United States-Mexico Agreement, Bothwell said the process is unlikely to be smooth.

"The CUSMA negotiations will be performative, as Trump attempts to extort the maximum concessions and to make it as public as possible," he said.

"It is possible that (Trump) and his associates will offer a commercial union, i.e. no internal tariffs but a common external tariff. You can guess who would set that tariff," he added.

Such an arrangement, he argued, would fundamentally undermine Canada's political system.

"It would therefore transfer a large part of Canadian sovereignty and democracy not merely to the US but to someone who sets tariffs according to personal whim — a return to governmental forms last seen in the Middle Ages," Bothwell said.

While rejecting that such proposals could carry economic costs, he said the alternative would be untenable. "The consequences of refusal will be unpleasant, but I do not see how any Canadian government could agree."

Even if the current aircraft dispute is resolved, Bothwell said further trade conflicts are all but inevitable. Asked whether similar frictions would continue to emerge, he replied simply, "Of course."

In historical terms, he said, the repeated use of tariff threats marks a dramatic deterioration in bilateral ties.

"Canadian-American relations are at their worst since at least the 1880s if not the end of the last war in 1814," he said.

gaoyang@chinadailyusa.com