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.
@evan number 4. Remove a couple of zeroes otherwise you'd have to evacuate Hamilton, Toronto, Ottawa, Montréal, and a whole lot of suburbs.
@Milnoc yes, that is entirely the point. The majority of the Canadian population is far too close to the border. We have to evacuate or we'll be entirely occupied within hours.