OTTAWA—Canada formally filed two complaints Friday that its best friend, neighbour, trading partner and defence ally, the United States, was an international trade scofflaw.

Right up to the last hours before Donald Trump’s announcement Thursday, Canada’s prime minister and his top advisers on the NAFTA file didn’t know whether the U.S. president would really proceed with his threat to slap tariffs on this country.

Should they take his June 1 deadline seriously?

Trump had changed his mind twice before. Each time it went down to the wire, a reprieve coming just hours before tariffs were to take effect.

Canada — along with the European Union and Mexico — had been exempted in early March, when Trump first moved against America’s global competitors with an eye on boosting domestic manufacturers. Another extension was granted in April.

Trump had imposed a 25 per cent tariff on steel and a 10 per cent tariff on aluminum imports on countries ranging from allies like Australia to nations he views as unfair traders, like China. Using a little-known provision from a 1962 trade law, Trump cited U.S. national security interests to curb imports, and in the process, trade lawyers said, thumbed his nose at widely understood international trade rules within the World Trade Organization.

In Canada’s case, Trump and his officials had made clear it was because little progress was being made at the NAFTA table.

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Foreign Affairs Minister Chrystia Freeland said officials had been working on Canada’s retaliation list “for weeks.”

In fact, after Trump fired his first warning shot, experts in the departments of Finance, Global Affairs, and Innovation, Science and Economic Development began secretly drafting Canada’s list of countermeasures to exert maximum pressure on key American congressional districts and power-brokers in Congress.

The government had conducted a savvy, yearlong NAFTA outreach effort to consult industry and union stakeholders, as well as important Opposition opinion-shapers like former interim Conservative leader Rona Ambrose and cabinet minister James Moore — but now, Ottawa could not afford to consult anyone on the list, keeping the retaliation plan in its back pocket.

It was a week that tested Trudeau’s own positions on two high-profile and high-stakes issues.

The Liberal leader who vowed to end federal subsidies of the fossil fuel industry, and who once mused about the need to “phase out” the oilsands, announced plans to buy an oil pipeline.

Then Trudeau, who went into office pledging to repair diplomatic rifts and who had won a measure of respect in the White House for his diplomatic dealings with Trump, found himself in the position of unleashing Canada’s toughest trade action in the postwar period — on a neighbour and Ottawa’s closest ally.

Both are monumental decisions. And both promise to have significant, though still uncertain, impacts on the country’s economy and his government.

Trudeau personally called the premiers of the provinces at the heart of each move: B.C. and Alberta, in the case of the Trans Mountain pipeline purchase, and Ontario, Quebec and Saskatchewan on trade tariffs. He reached out as well to brief opposition party leaders Andrew Scheer and Jagmeet Singh.

The Conservative leader, Scheer, was quick on Thursday to fault Trudeau in the trade battle, declaring that the prime minister had “failed” Canadian steel and aluminum workers. Conservative MPs took the same tack in Friday’s question period.

But other prominent Conservatives pointedly broke ranks and voiced support for the Liberal government’s response.

“We must defend our economy and hit back,” Rona Ambrose, the former Conservative leader, said on Twitter.

“When Canada is under attack, we unite. Country first,” former cabinet minister James Moore tweeted, calling the government’s actions “measured and appropriate.”

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Freeland singled out some of those voices, along with those of union leaders, as she claimed broad support for Canada’s retaliatory response.

“This is a moment when it is important for us to be clear and firm,” she said.

But uncertain days lie ahead, said Roland Paris, professor of international affairs at the University of Ottawa, who served as Trudeau’s senior adviser on global affairs and defence.

Uncertainty over the NAFTA negotiations, worries about further trade action that could target Canada’s auto sector, and, above all, a president who seems less restrained in his actions and rhetoric create a troubling picture for Canada, Paris said.

“He’s previously said that trade wars are easy to win,” Paris said, referring to Trump. “Well, he’s putting that proposition to the test and we’ll see how much ground is scorched in the meantime.”

Trudeau’s challenge with Trump has always been to strike a balance between defending Canadian interests and avoiding an escalation of tensions, Paris said.

“I think the balance point has shifted but I think his approach to dealing with the Trump administration will continue to be careful and calibrated,” he said.

He cited Canada’s response to the U.S. tariffs, calling the dollar-for-dollar action “commensurate.”

“I think that’s part of not wanting to escalate these tensions any further, not rise to the bait of Trump’s various tweets,” Paris said, adding that it is “profoundly” in Canada’s interests to keep the trade spat from spiralling further.

He said the trade measures announced by Washington are likely part of a larger gambit by Trump to exact trade concessions from Canada and the other nations, and the president cares less about the diplomatic carnage it might create.

“Previous presidents would have been more restrained at the risk of doing serious damage to these relationships or to the international trading system,” Paris said.

“The worse-case scenario for most presidents doesn’t seem that bad for him.”

If there is a silver lining, Trump’s seeming mercurial nature means that he could just as quickly call a truce in the trade war.

“He can overnight shift from being adversarial to friendly,” Paris said.

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