So, here's my defense plan for Canada. Basic philosophy: it is unsafe to wait for an attack.
1. Get public confirmation from NATO that Article 5 applies even if the aggressor is also a NATO member.
2. Send an ultimatum to Washington demanding a public acknowledgement of Canadian sovereignty by the President and confirmation of non-aggression.
3. In the absence of that acknowledgement, sever diplomatic ties, close the borders, and embargo trade. Blow bridges, tear up roads and rail lines.
4. Evacuate Canadian civilians from the border area; probably 300km or more. Yes, this is where most Canadians live.
5. Declare a security corridor of 300km on the other side of the border, in US territory. Any military activity in that area is a sign of imminent aggression and will prompt a defensive strike.
6. If anything occurs, surge forward and take territory. Keep any war on US soil, not in Canada.
The goal is to get Canadians out of harm's way for a shooting war with short-range missiles (500km-1000km); keep something like an economy running, although severely curtailed by the loss of US trade and any facilities near the border; and bring the maximum pain to the US economy, civilian morale, and government.
We can't afford giving an aggressor the benefit of the doubt; too much of our population is within an hour's drive of the border.
If we wait until the US military moves into position to invade, we will have already lost.
We have thousands of kilometers of borderland between the continental US and Canada, as well as Alaska and Canada. Even if the US makes headway into Canada, we can identify areas of the US to occupy. The psychological effect of having US territory occupied by a foreign country would be really devastating on its citizens.
@evan While fighting on enemy territory may be the correct military strategy, I'm not convinced taking enemy territory will inevitably have a significant effect on enemy morale. Ukraine advancing into Russian territory doesn't seem to have crushed Russian morale; or, alternately, Russian morale is already shot and the war machine does not depend on the morale of Russian soldiers or civilians.
@skyfaller nothing's inevitable. Americans are daintier than Russians, though. Surviving occupation is the national sport in Russia, but Americans haven't had to do it for 250 years.
@skyfaller I don't think military plans are about finding One Weird Trick to Avoid Invasion. It's about finding weaknesses and exploiting them the best you can. I think occupation is a real weakness, and keeping the fight in US soil is also important.
Military planning is to identify goals and accomplish them in ways most surprising to the enemy.
Ideally, with no one hurt.
I like the first part of your plan the most: address this through the already extant treaties of international law - WHICH ARE ALSO US LAW. (See US Constitution, Article III, section 2)
@Amgine How long do the USA last without Canadian oil?
https://www.eia.gov/todayinenergy/detail.php?id=62664
https://en.wikipedia.org/wiki/Strategic_Petroleum_Reserve_(United_States)