I'm not sure about this. They've got a more than capable military.

But I'm willing to find out where their limit is. Militarily, I suspect it starts at a protracted war of attrition with a determined enemy that looks exactly like them and who pops in and out of unassailable and impassable northern boreal forests and muskeg swamps. Or hides in plain sight in the middle of Toronto. Or hops over the immense border to wreak havoc. No one would be safe, ever.

Economically, I think cutting off hydro, oil and potash would have an enormous effect on their comfort level. For lasting damage, nationalize all strategic resources and seize assets and companies. Raze them if threatened, starting with US assets. Nationalize all IP and patents. Sell all US government debt and send the interest rates sky high.

The weakness the US has is that they are doing this for more power and a comfortable life. They AIM to survive it. Frankly at a certain point I don't care about that. Our fight is existential, and we have given up on a vision of life after. What matters at that point is the here and now, and that's it.