Canada

If Canada is at all serious about fighting a conventional war with the US you would need to start by moving your population and all critical infrastructure outside of ATACMS range of the US border. The reality is that if the US were to decide to mount a full-on invasion Canadians would learn when everything important blows up.

Nuclear is only a deterrent if the adversary can't be certain of taking out your nuclear capabilities in a surprise first-strike. This was the logic of the nuclear triad: You need nukes on land, nukes under the sea and nukes in the air so if the enemy strikes you still have nukes to hit them back.

The US has 50 attack submarines. Any subs Canada could acquire would have multiple attack subs on them in a hot war.

A much more realistic (and less costly) approach to security would be recognizing and focusing on Canada's considerable soft power, and understanding how the US works and what limits its ability to act. For instance, Bush needed Blair for the Iraq war because he needed to look like the head of a coalition. Blair needed the UNSC vote. If the UNSC vote had not gone their way, the war would not have happened.

If the US were to attempt a war with Canada it would have to start with a big preamble of kabuki theaters and drum pounding. The US is incapable of acting without a big, loud public pressure campaign. There will be thousands of chinks in the armor of the "war case" that Canada would have to exploit. And there would be "pressure points" like the UNSC vote in Iraq where Canada would have the opportunity to halt the whole thing if its diplomatic power is strong enough.

If Canada really was resigned to being attacked from the south and wanted a realistic way to present as being too costly to fight even by a despotic Putin-like regime then they would have to focus on proving that, even though the conventional phase of the war would inevitably be a walkover, the peace would be unwinnable. Canada would have to present itself as a patriotic, cohesive, determined nation of potential partisans.