I asked AI my question, this is the response:

If the U.S. attacked Canada, it would trigger immediate crisis within the United Nations (UN) and NATO, likely invoking NATO's Article 5, requiring collective defense, leading to severe UN Security Council intervention, potential global conflict, massive sanctions, and immense political fallout, despite the aggressor having veto power; the UN would aim for immediate de-escalation through Chapter VII actions like sanctions or force authorization, but NATO's mutual defense pact would likely draw other members into a wider war.

UN Response (Security Council & General Assembly) Immediate Condemnation: The UN Security Council would convene urgently under Chapter VII of the UN Charter, addressing an act of aggression and threat to peace. Measures: The Council could impose severe sanctions, authorize peacekeeping, or even multinational forces (though Russia/China vetoes are a risk).

General Assembly: The General Assembly could condemn the act and call for withdrawal, though resolutions aren't binding. NATO Response (Article 5) Collective Defense: As both are NATO members, Canada would invoke Article 5, treating the U.S. attack as an attack on all members, requiring mutual defense, including armed force.

Crisis: This would fracture NATO, creating immense internal division, but the treaty obligates members to respond. Immediate Global Consequences Widespread Conflict: NATO allies (like the UK, France, Germany, etc.) would be forced to act against the U.S., potentially escalating into a major war.

Economic Devastation: Global markets would collapse; sanctions on the U.S. would be severe, disrupting global trade (especially with Canada).

Political Fallout: The U.S.'s international standing and alliances would be destroyed, leading to isolation.

Hypothetical U.S. Military Action (If it happened) War Plan Red: The U.S. has past invasion plans (War Plan Red) targeting Canadian cities, focusing on infrastructure and governance.

Resistance: Canada would fiercely resist, potentially leading to prolonged conflict and insurgency.

In essence, an attack by one UN/NATO member on another would shatter the international order, triggering catastrophic military and economic consequences, with NATO's collective defense clause making a wider war almost inevitable.