Why Invading Canada is Justified According to Gary Becker:
preventive war is justified because of the potential severe consequences of waiting until attacked to apprehend.
Preventive: 2. Carried out to deter expected aggression by hostile forces.
Hostile: 2. Feeling or showing enmity or ill will; antagonistic.
So, given what we know about quantum weirdness, let alone geopolitics, the probability of a Canadian use of a non-zero amount of force against the US is non-zero. Therefore, the expectation of the amount of force being used against us is non-zero. Therefore a preventive invasion of Canada is justified according to the formulation of Becker (Jew).
Perhaps a superior formulation would be that pre-emptive war is justified against a hostile entities.
Pre-emptive: 3a Relating to or constituting a military strike made so as to gain the advantage when an enemy strike is believed to be imminent.
Imminent: About to occur.
About to: Ready to, on the verge of,
Because Canada has not been observed to be at all prepared to invaded the US and because there would be several intervening steps which the Canadians would have to take (like troop mobilization) in order to attack the US, Canada is neither ready to nor on the verge of attacking the US.