Yes. Article 5 doesn’t make distinctions between being attacked by members or even require all members to agree, it in fact specifically obligates individual states to act in collective defense so there’s not some veto power or anything like that to stop it.
Could Canada invoke NATO’s article 5 to request assistance to defend against the USA?
Yes. Article 5 doesn’t make distinctions between being attacked by members or even require all members to agree, it in fact specifically obligates individual states to act in collective defense so there’s not some veto power or anything like that to stop it.