US achilles heel is intellectual property
The US has an achilles heel in that Canada can, if NAFTA is scrapped, nationalize virtually every US tech and pharmaceutical patent and probably most of their software copyright as well. And then use those technologies (often produced by Canadian universities in part or in whole) to sell services, copy software, produce Teslas (calling them Theslas or something like that) and all forms of medication produced in the US, without paying so much as a cent to the rights holders. It could easily devastate 10-20% of the US economy. Then there's severing US access to critical minerals from Canada. We can develop industries and sell those resources to, say, China, Korea, Japan, the Eu, etc., and cut the US access off. Finally there's water. Canada can cut a huge amount of the water the US desperately needs from the Colorado by simply "turning off the taps" (as Trump would put it), shrivelling the US Western 1/3 of the country like a dried out prune within weeks. If the US tries to seize the water, Canada simply poisons it with nuclear waste since the water flows into the US, not back from the US