A historical aversion to missile defence has left Canada unshielded, as North Korea and the United States rattle their sabres.

In the short term, this may not matter much. In the long term, analysts say, Canada needs to get under the protective American umbrella.

"Before you could argue there was no real threat, so why are we doing this?" observes Bill Graham, who was foreign affairs and then defence minister in the Paul Martin governments. "But the threat dimension has changed. This is a genuine, real threat."

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A brief historical primer: In 1957 Canada joined with the United States to create NORAD, the North American Air (now Aerospace) Defence Command, which is charged with detecting an airborne attack against the two countries.

But while Canada was willing to work with the U.S. in detecting incoming threats, it was reluctant to join any system to eliminate those threats. In the early 1960s, prime minister John Diefenbaker refused to permit the use of nuclear warheads on the recently deployed Bomarc anti-aircraft missiles. After the Conservatives were defeated, Liberal prime minister Lester Pearson permitted the nuclear-tipped Bomarcs, but Pierre Trudeau later phased them out.

In the 1980s, Conservative prime minister Brian Mulroney declined Ronald Reagan's invitation to join the Strategic Defence Initiative, commonly known as Star Wars. Many military strategists strongly opposed such anti-ballistic missile defence systems (ABMs), because they undermined the principle of Mutually Assured Destruction (MAD). MAD deterred any country from launching a nuclear first strike, because it knew the other side would have enough surviving missiles to launch a counterstrike.

An ABM system, critics charged, could tempt one country to launch first, hoping that its missile defence system would protect it from retaliation, thus increasing the risks of nuclear war. The Canadian government opposed ABMs for that reason.

"It's a lingering Cold War mentality," says Joel Sokolsky, a political scientist at the Royal Military College in Kingston. But that mentality doesn't apply to North Korea, which is seeking to acquire nuclear weapons to deter an American attack on the regime.

"In this case, the nuclear weapon is the weapon of the weak, not the strong," he says. To protect against a Korean attack, he believes, a missile defence system is both useful and necessary.

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More than a decade ago, George W. Bush's administration decided to construct a ballistic missile defence system (BMD) to protect against a limited nuclear strike on North America, and asked for Canada's support.

Liberal prime minister Paul Martin initially supported Canadian participation in BMD, But as the Bush regime became steadily less popular thanks to its deceptive and disastrous involvement in the Iraq War, the idea of joining BMD also became steadily less popular, especially among younger voters and in Quebec.

In February, 2005, Canada announced that it would not participate in the BMD program. As defence minister, Mr. Graham regretted the decision then, and regrets it still.

"The ask was minimal and the opportunity was great," he recalled in an interview. The Americans wanted no financial contribution and weren't asking to locate weapons on Canadian soil. But domestic politics trumped strategic interests.

If, some time in the next few years, North Korea becomes capable of launching nuclear-tipped ICBMs at the United States, Canada will have no say in the deliberations over how the missile defence system might respond to an attack, even if one or more missiles are headed, by accident or design, for Canada.

That might not matter. There is near-universal agreement that the United States would seek to shoot down a missile directed at Vancouver, Toronto or Montreal, if only because of their close proximity to the United States.

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"The good news is that most of Canada lives where most of the U.S. lives … so we're going to get some partial coverage," says Andrea Charron, who is deputy-director of the Centre for Defence and Security Studies at the University of Manitoba.

Nonetheless, if North America were subjected to an attack involving numerous missiles, one or more of which targeted Canada, there is no guarantee American military commanders would defend Canadian territory, absent a joint missile-defence agreement.

And here's a thing: Canada supports the new missile-defence shield protecting its NATO allies in Europe. As Prof. Sokolsky points out, that means Canada supports missile defence for every country except Canada.

"Our attitude toward BMD is viewed with more bemused curiosity than anger in the United States," he says.

The defence policy review that Defence Minister Harjit Sajjan released in June does open the door to discussions between Canada and the United States in co-operatively developing the next generation of air defence technology. Mr. Graham, Prof. Charron and Prof. Sokolsky all believe the Canadian government should consider expanding those discussions to encompass a Canada-U.S. BMD system, which could take the form of making a financial contribution to the American BMD system, positioning missile detection facilities on Canadian soil, or even having Canada host a battery of anti-missile weapons.

That said, the Americans may no longer be interested in Canada's participation, said Prof. Charron. And in any case, "there are limited things that Canada can do," because "there are limited things that the U.S. can do." No one has high confidence that the U.S. missile defence system would have anything approaching 100 per cent accuracy in shooting down incoming missiles.

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And "at the end of the day, they only have so many [anti-missile weapons] that they can shoot." Prof. Charron said. "And if North Korea has N plus one …"