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.

U.S. President Donald Trump has put a price tag on Canada joining his proposed Golden Dome missile defence system — and renewed his annexation threat in the process.

Canada has a large airspace that offers the US an important line of sight for the US sensors to shoot down missiles from China and Russia. Moreover, historically, Canada has played a big role in air and missile defence for North America.