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It was in a speech in Halifax in December 2004 that a newly re-elected George W. Bush—worried about the nuclear threat from North Korea, Iran and from terrorists—pitched Canadians on a continental missile defence shield “to protect the next generation of Canadians and Americans from the threats we know will arise.” Paul Martin, then the prime minister, rebuffed him, to the relief of his fractious caucus.

Now Stephen Harper’s government is reconsidering whether Canada should join in the controversial program as part of its national defence strategy.

Since 2005, 30 interceptor missiles—the 17-metre-tall infrared-guided rocket built to collide with enemy warheads 150 km above Earth—have been installed in America, most in Alaska with a handful in California. Barack Obama’s administration has said it’s studying the addition of a third cluster in the eastern U.S. (This is not Ronald Reagan’s 1980s’ “strategic defence initiative,” known as Star Wars, that envisioned the weaponization of space. Today’s system relies on ground- and sea-based interceptors that do not carry nuclear material.)

Already through the binational command at NORAD, Canadians share information in early warning and attack assessments with the U.S. But advocates of the system complain Canada doesn’t have a say in responding to a potential missile attack. “It seems ludicrous, but when it comes time to make the critical launch decisions, our officials literally have to leave the room,” says Colin Robertson, a former Canadian diplomat in Washington and a fellow at the Canadian Defence and Foreign Affairs Institute, who plans to testify on the subject before the joint committee on defence in Ottawa on May. 8. Robertson says Canada should benefit from the U.S. security umbrella. “The algorithms developed by U.S. Northern Command to protect the American homeland do not include Canadian cities like Edmonton or Saskatoon.” (Some Canadian cities benefit from proximity to large U.S. centres.)

Canada has already endorsed missile defence internationally. In 2010, all 28 countries of the NATO alliance agreed to “develop the capability to defend our populations and territories against ballistic missile attack as a core element of our collective defence.”

The vulnerability of North America, however, is open to debate. In 2013, North Korea boasted it could attack the U.S. mainland. “They probably can’t—but we don’t like the margin of error,” said Obama, who responded by ordering up 14 new interceptors by 2017. As Iran acquired short and medium-range missiles that could reach Europe or the Middle East, Obama placed interceptors aboard navy warships that could be closer to Iran.

Now Russia’s aggression against Ukraine has renewed interest in bolstering missile defences in Europe, including demands to speed up the deployment of missile interceptors in Poland and Romania.

What role Canada could play in the system by “joining” it is unclear. (The U.S. has not pressed Canada to contribute and in 2004 it didn’t ask for financial support or to put interceptors on Canadian soil.) The system’s critics are still numerous. Missile defence “has yielded a dysfunctional weapon system and has set back efforts at nuclear disarmament,” says former Canadian diplomat Paul Meyer, a fellow in international security at Simon Fraser University.

When the U.S. sent a missile destroyer ship to Spain last month to “reassure allies” in the wake of Russia’s annexation of Crimea, Moscow said it proved false Washington’s past assurances that the sea-based interceptors were aimed only at Iran. “It all confirms our previous estimates that the missile shield in Europe is aimed at undermining Russia’s nuclear deterrent,” said Russian deputy defence minister Anatoly Antonov.

But the clearest obstacles are technical. Only half of test launches have succeeded in intercepting their practice targets. “Neither North Korea nor Iran have [intercontinental ballistic missiles] that could threaten North America. The current interceptors would be useless against them even if they possessed such missiles,” says Meyer. Thorough testing of the system won’t be completed “until at least 2022,” according to the U.S. Government Accountability Office. But the Harper government can’t wait until then to make a decision.