Canada has 51 trillion USD, actually a bit more than that in assets. It is very weak, militarily and would be defeated by the US within 48 hours, bar some limited guerilla activity from the remnants of the Candian military that survive the initial invasion. As regards the rest of the world, European allies like the UK, France, and Germany would face an existential choice. They can freeze US assets, expel US personnel from their military bases and sever intelligence sharing agreements. Indeed the invasion of Canada would collapse the post-WWII international order, fundamentally realigning global alliances. However, as we saw with Russia, NATO is even reluctant to fight the Russians. It is to be expected that the UK, France and Germany would be even more reluctant to fight the USA. NATO of course would be destroyed. Does the US really need NATO though? It is really the other states that need the US in NATO, not the other way around. All the UK, France, Japan, Germany etc could do is impose economic sanctions on the US. However, arguably the 51 trillion USD the US would gain by invading Canada, would cushion the economic impact quite nicely. The US could wipe out all its national debt with Canada's assets. And there would be the added bonus of seeing Doug Ford in handcuffs.
