It has been hotly debated, mostly in regards to Greece and Türkiye, who have been at each other’s throats for decades. In that case, the aggressor likely wouldn’t be obvious and both are likely to claim article 5. Their relations to other NATO members are also a lot more ambiguous, some favoring one, some the other.
In the case of a rogue USA attacking Canada, or Denmark, I believe the case would be a lot clearer.
It has been hotly debated, mostly in regards to Greece and Türkiye, who have been at each other’s throats for decades. In that case, the aggressor likely wouldn’t be obvious and both are likely to claim article 5. Their relations to other NATO members are also a lot more ambiguous, some favoring one, some the other.
In the case of a rogue USA attacking Canada, or Denmark, I believe the case would be a lot clearer.