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Former prime minister Stephen Harper speaks at a gala commemorating the 20th anniversary of the Harper government in Ottawa, on Wednesday.PATRICK DOYLE/The Canadian Press

Stephen Harper said in remarks on Wednesday evening that Canada must protect its industrial capacity in coming free trade talks with the U.S. and be prepared to hit America with tariffs if necessary.

But the former prime minister also warned against Canada blaming all of its economic pressures on U.S. President Donald Trump, saying past government decisions are also to blame.

Mr. Harper made the remarks at the tail end of a speech at Ottawa’s Rogers Centre, during a gala celebrating 20 years since the Conservative Party first formed government.

Speaking at other events this week, the former prime minister has emphasized the need for national unity and for political parties to work together as the country’s sovereignty remains under threat.

On Wednesday night, he offered more tactical advice.

Mr. Harper, who now runs a consulting business, said it’s not a safe assumption that in the U.S. “things will go back to the way they were in due course.”

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Canada must adjust to new global realities, and focus not on how the country feels about U.S., but how it will adapt to changes there.

That includes taking a hard look at past decisions, he said.

“Our national conversation must be mature enough to acknowledge that many of the difficulties we now face as a country cannot be blamed on Donald Trump,” he said.

“In too many cases, they stem from bad decisions by our own government.”

But, he said, the Trump administration is a serious challenge, and Canada must reduce its dependence on the U.S. while still recognizing that, by all measures of international relations, America remains Canada’s principal partner.

“Thus our future path requires balance and sober reflection,” he said.

In the coming talks on the United States-Mexico-Canada free trade agreement, he added, Canada must defend two things: its industrial capacity and its potential to sell resources to world markets.

Canada must fight against any attempt by the U.S. to bring manufacturing south, he warned.

“One-way tariffs introduced by the U.S. must be reciprocated,” Mr. Harper said.

“Now, will that be costly in the short run? Yes, but what we can never concede are permanent, one-way economic flows from Canada to the United States.”

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Mr. Harper also said Canada must focus on its potential to find new markets for natural resources – and get government out of the way.

“Canada needs more than a list of major projects,” he said, a reference to an initiative by Prime Minister Mark Carney to speed up resource development.

“We need to reinstate the reforms our government enacted to streamline the regulatory process and deliver decisions in a reasonable time frame.”

Mr. Harper further called for the immediate construction of a new oil and gas pipeline, saying it was a matter of economic urgency and national unity.

“There is simply no way to explain to people in my part of the country why a nationally beneficial project is singularly blocked by federal policy,” he said.

Mr. Harper lives in Alberta.

His speech Wednesday night was before an audience of hundreds of former staffers, and past and present conservative politicians, including current federal party leader Pierre Poilievre.

The event was sponsored by Mr. Harper’s firm, as well as a range of corporations and businesses. Attendees had to purchase tickets in order to attend.

The evening also included tributes to Mr. Harper’s leadership and time in office from his former colleagues in Ottawa, as well as former British and Irish prime ministers and former U.S. President George W. Bush, who sent a video.

Mr. Harper said a memoir of his time as Prime Minister is to be published later this year.