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.
Another principle is jumping before we are pushed. if we wait to let troops and ordnance move to the border, while we hope to preserve trade with some last-minute deal, we're done. We have to take the initiative in our own defense or we will be subject to the whims of Washington.
Last note: I am a software developer and standards enthusiast, not a military planner. I hope that PM Carney and his cabinet are having aides develop much, much better plans at this level of seriousness right now. I'm sure theirs will be a lot better than mine.
Oh, one more thing: a common strategy for invaders is to let domestic rebellions happen, and then come in to "protect their rights". Hopefully CSIS and the Mounties can come down hard on any Canadian going on Fox News to invite invaders to come annex our country. It's a valid political position and thought experiment in peacetime; when there is a real possibility of a war, it's treason. Straight to jail! And definitely don't allow any kind of trucker convoy demonstrations in large numbers.
One more, more thing: China.
One temptation would be to get military support and other aid from China. It's a huge economy, would benefit from our economic resources, and has massive military resources.
I think this might backfire badly. It would go from a war of choice for the Americans to a war of survival. They would probably fight more desperately than if it was just Canadians and their European allies.
@evan Someone had to say it.
“You fell victim to one of the classic blunders! The most famous of which is, ‘never get involved in a land war in Asia,’...”