Neither European country is an approach to the US. If the US determined that American boots needed to be on Canadian soil to counter an imminent and significant threat, they are more likely to tell us what they are doing than to ask our permission.
That being said, it is difficult to imagine what hypothetical threat might actually trigger such a behaviour so long as we remain a contributing, cooperative partner in NORAD.
The calculus changes under a Trump admin, as Edward mentions above. The MAGA camp has already pontificated about using military force against Canada and about “liberating” Canada. It would possible to see a Trump administration conflating failure to meet the NATO 2% GDP commitment with a failure to meet continental defence obligations (it would not be unreasonable for any administration to connect these two things despite there being technically separate). It would not subsequently be completely improbable to see a Trump administration leap to the conclusion that America must occupy the delinquent Canada to secure itself.