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President Donald Trump makes an announcement about the Golden Dome missile defence shield at the White House in Washington in May.Kevin Lamarque/Reuters

Mark Carney’s decision to work with Donald Trump on an advanced missile shield – what the President calls the Golden Dome – will set the stage for deepening military co-operation between the two countries.

It’s a stark difference from 20 years ago, when Canada opted to stay on the sidelines as the United States developed and bolstered defences against ballistic missiles – measures that today include ground-based interceptors in Alaska and California.

In February, 2005, Prime Minister Paul Martin’s minority Liberal government publicly announced that it would not participate in President George W. Bush’s ballistic missile defence (BMD) program. Critics at the time had warned that it could lead to a new arms race, and Ottawa cited the Americans’ refusal to promise that the plan would not lead to the placement of weapons in space.

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Former Liberal defence minister David Pratt, who served in Mr. Martin’s cabinet before the MP was defeated in the 2004 election, was involved in early consultations with American officials on whether Canada should join.

He considers the 2005 decision a mistake, calling it a “short-sighted, politically motivated” call. The Liberals, Mr. Pratt said, were getting pushback from some in their own caucus as well as the NDP. “They were in a minority situation, so they had to tread very carefully, and they didn’t want to die on the mountain of ballistic-missile defence.”

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U.S. President George W. Bush and Prime Minister Paul Martin in Gleneagles, Scotland, in 2005. Mr. Martin’s government decided not to participate in Mr. Bush’s ballistic missile defence program.Jason Reed/Reuters

Mr. Pratt said the decision rendered the country a “second-class member” of NORAD. It meant Canada, and NORAD, were limited to providing aerospace surveillance and warning for missiles but were excluded from interception roles for ballistic threats. This ended up marginalizing Canada and NORAD as other American commands such as Northern Command took over this role.

“Our situation got downgraded so that we were not going to be in a position to be in the room when decisions were taken,” Mr. Pratt said.

Today, potential missile threats include North Korea, Iran and countries that for a brief time in the 1990s and early 2000s appeared to be potential partners instead of rivals.

Last year, Canada’s top soldier, General Jennie Carignan, told reporters that she thinks Canada and the United States have five years to prepare for new threats from adversaries such as China and Russia. Both countries are developing new hypersonic missiles that can fly five times the speed of sound and change course midflight.

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Michele Cadario was deputy chief of staff to Mr. Martin in 2005. She said that 20 years ago was a different time. Vladimir Putin’s Russia was a member of the Group of Eight. He had not invaded Ukraine. There was not a war being waged in Europe.

“I think it’s really hard to kind of take the decision that was made then, given the circumstances, and try and transpose that to 2025,” Ms. Cadario said.

Mr. Martin himself said in a 2017 interview with Global News that his rejection of ballistic-missile defence was based on information available at the time – and he might make a different choice today.

Ms. Cadario said the feeling in Ottawa in 2005 was that BMD “was more of an untested technology” and that specifics were lacking from the Americans on what Canada’s role was going to be and how it would work.

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An artist’s concept of a space missile defence system from around 2000. It shows a satellite system and defence radars in western Europe and the U.S. guiding rockets to destroy incoming warheads.Science Photo Library

Vincent Rigby, a professor at McGill University’s Max Bell School of Public Policy who in 2005 was director-general of policy planning at the Department of National Defence, said the benefits at the time to Canada from joining the U.S. on missile defence weren’t clear.

Prof. Rigby, part of Canada’s negotiating team in the BMD talks, said the United States was primarily interested in political support rather than access to Canadian territory, although it was an open question whether that could change in the future.

“The American answers were often vague, if not downright ambiguous,” he said.

“The Americans could not give us a definitive answer on what the system was ultimately going to be able to give us in terms of protection.”

Prof. Rigby, who more recently was a national-security and intelligence adviser to former prime minister Justin Trudeau, agreed that the situation is very different today.

“You cannot walk away from the United States,” he said. “Geography is not going to allow us that.”

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David Perry, president of the Canadian Global Affairs Institute think tank, said the problem is that Canada could also be left vulnerable to missiles if it remains outside the shield when the U.S. is scrambling resources to intercept incoming threats.

Saying no to BMD in 2005 meant, “We were no longer involved in activity to defend the continent against a key threat and we stopped having insight into what the U.S. was doing in that respect.”

Eight years ago, the top Canadian in charge at NORAD painted it more starkly, telling a Commons committee that the United States was under no obligation to defend Canada in the case of an intercontinental ballistic missile attack.

“We’re being told in Colorado Springs that the extant U.S. policy is not to defend Canada,” then-Lieutenant-General Pierre St-Amand, deputy commander of NORAD at the time, told MPs. “So that’s the fact I can bring to the table.”

Mr. Trump’s grand vision of weaving multiple air defence systems – radars, sensors and interceptors – into a single networked command and control system to improve overall efficiency and effectiveness is an attempt to replicate what Israel has done on a much smaller scale with its Iron Dome. That vision draws inspiration from Ronald Reagan’s never-built space defence program, nicknamed “Star Wars.”

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A silo housing an interceptor missile at the Fort Greely missile defence complex in Alaska in 2018.MARK MEYER/Reuters

The Trump administration projected a cost of approximately US$175-billion over three years to develop and deploy Golden Dome. However, the Congressional Budget Office estimated a range of US$161-billion to US$542-billion over 20 years.

Former Conservative defence minister Peter MacKay, a member of Stephen Harper’s government that succeeded Mr. Martin’s and that also declined to join the U.S. ballistic missile defence program, supports participating in Golden Dome.

Nevertheless, Mr. MacKay said there are lots of serious, unanswered questions about Golden Dome, which ambitiously aims to link together all existing missile defence measures and put interceptors in space.

“We’re a long way away from being able to replicate the Iron Dome of Israel,” he said. “We’re talking about putting a dome over the entire northern half of this continent, which is something clearly that’s never been done before. The order of magnitude, the technology – which doesn’t yet exist – the level of investment, this all goes beyond the limited discussion that took place around Star Wars, you know, and the alarmist rhetoric around putting missiles in outer space.”

Technology may be closer than ever to creating this, Mr. MacKay said, but it’s also prohibitively expensive, “and it is also something that, from a political standpoint, is very, very contentious and frightening for a lot of Canadians.”

He emphasized that Canadians are anxious about it.

“I think the idea of putting weapons in space is very jarring for a lot of Canadians, because I don’t think people believe that the technology is trustworthy enough,” he said.

“People are still wrestling with the impact of artificial intelligence in the defence community, so putting missiles in outer space, let alone nuclear warheads, I think, is very much a bridge too far right now.”

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Prime Minister Mark Carney and U.S. President Donald Trump at the White House on Tuesday.Adrian Wyld/The Canadian Press

Canada has already pledged $40-billion over two decades to modernize NORAD, including new radars and sensors. And Mr. Carney has pledged to add tens of millions to annual military spending as part of a new NATO commitment made this June.

James Fergusson, a senior research fellow with the Centre for Defence and Security Studies at the University of Manitoba, said it’s likely that Canada will have to spend more than its NORAD modernization commitment as a contribution to Golden Dome.

Mr. Pratt said Canada must engage with the U.S. on missile defence, but at the same time needs to determine precisely what this country can afford “and what we wish to protect.”