Canada is in NATO, and attacking Canada would automatically put you at war with the USA. You cannot consider Canada's defense policy independent of the USA.
[1] http://www.fallen.io/ww2/ (Skip to the 14 minute mark if you're impatient)
Interestingly, this convention is signed and ratified basically by the whole world, but not by the US.
[0] https://thebarentsobserver.com/en/arctic/2019/04/russia-scor...
[1] https://en.wikipedia.org/wiki/United_Nations_Convention_on_t...