The president would need to rely on Ottawa to track Chinese and Russian missiles that could come at the U.S. from over the horizon.

Some experts are worried about the cost and timeline of U.S. President Donald Trump's proposed "Golden Dome" missile defence system, which Canada has expressed interest in joining. They also say it could set off a space-based arms race that would escalate global tensions.

PM Mark Carney's office says there are "active discussions" on the Golden Dome, but it's too early to say how Canada would join or pay in.