The president would have to rely on his “51st state” 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.