It would be a shit storm of unimaginable proportions. The US would be attacking another NATO nation and Canada would almost certainly invoke Article 5. Those not assisting militarily would probably sanction the shit out of the US and many of their previous treaties, pacts, and agreements would be withdrawn.

Recent polling in the US has shown that US citizens are greatly opposed to the ongoing trade war with Canada and would be even more opposed to further conflict. The executive branch ordering an attack on a historic ally and neighbour would cause catastrophic rift in the US military chain of command and I'd imagine many would disobey orders. States along the Canada/US border would likely be fiercely opposed to having their states turned into battlegrounds and would resist legally in whatever way they could.

Canada does not have the military for a large prolonged conflict, especially from our closest ally. An actual war would probably be quite shortlived, but it would also probably be the last thing the Trump administration ever does. The economic and civil-unrest blowback would rip apart his administration. Ordering your citizens to kill their neighbours, friends, and even family is not something you do without major consequences.

Domestic insurgency at home. Guerilla warfare in Canada.

America loses another war. We haven't won one in 70+ years. Should be used to it.

Well for one thing, you'll find out how many Canadians are willing to use a gun in defence of their country, despite very few being invested in the American gun culture.

Also, it would likely be the beginning of WWIII.

I really hope we won’t find out.

My guess is that the military command would probably refuse the order. Or at least some parts would refuse, some would follow, it would be chaotic and could easily trigger a civil war.

But if they all follow the order..?

I really hope there would be major unrest across all US cities. General strike and millions on the street. This could end the trump presidency relatively quickly. Something like in South Korea recently, just much bigger.

But if not?

From a military point of view, Canada and Europe wouldn't stand a chance. Unlike the Russians in Ukraine this would be a quick operation. However, the guerrilla war afterwards would be pretty damn fierce. But it would be a damn big shock for the global economy. The USA would suddenly be completely isolated. Global trade would probably grind to a halt. China would attack Taiwan. Organizations like the UN would be on the ground. The rest of the world would immediately try to arm itself with nuclear weapons. The global economy would collapse. Sooner or later we would have total chaos.

It would be the point of no return for the USA, and the conclusive end to the "Pax Americana" . Invading a peaceful neighbor? Unthinkable for a "so-called" stable country. The rest of the world rally to Canada's side, there would be an effort to 'contain' the further damage we could do. (Mexico would finally pay for that wall :-/ ) We would no longer be trusted, and in that vacuum of power the rest of the countries of the world would be thrown into such a frenzy to fill that power, or to align with who they think could.

I have a very hard time believing that the majority of US soldiers would be willing to attack Canadians.

I really struggle to see a unilateral mobilization and that the US military truly goes through with it.

Especially when foreign assets are getting absolutely left stranded by Trump, programs are getting gutted (he hasn't touched the VA, yet) and everyone who helped after 9/11 were getting shafted by their government in terms of Healthcare.

If you take away the rather potent motivators of "national security and safety or preventing terrorism" I think you're gonna get a lot more of a strike from soldiers. Unless you can somehow believably apply that to canada.

Keep in mind that Greenland and Canada are both members of NATO, and thus the US would be forced to intervene...

Oh, well...

You mean seriously? Who knows? Both sides have nuclear weapons, don't they? This is not something that ever happened. Would the US armed forces go along with the plan or will there be a coup? Will everybody keep their heads leveled and avoid exchange nuclear explosions even with people dying at home / on the front? Will somebody just put a bullet on Trump's head and stop the madness?

But the thing I can tell is that if you are expecting country lines and military treaties to be immutable, that's not how they work.

What would be the reaction of the Canada King (Charles III is the king of Canada)?

Absolutely nothing. Maybe some strong words. He has no power.

The Commonwealth? NATO?

Tough to say. Getting actively involved in a war in North America would be very difficult logistically for the European countries or the Commonwealth, so I don't think there would be an immediate military reaction. Possibly a Ukraine-type situation where the NATO countries try to aid Canada with equipment as best as they can. But even that will be difficult across the Atlantic ocean, which the US can probably blockade. Local politics would be absolute chaos, with an ally that many countries have relied on suddenly turning enemy. With the US having military bases everywhere, they could also threaten countries into keeping out of the war. I think the situation would unfortunately be quite bleak for Canada.

Practically speaking since war is unthinkable is would result in as much economic isolation as Europe can bear. It would be the end of NATO. Almost immediately there's be European voices saying 'What's the real harm?' and other appeasers. I think the political lash back would only last 5-10 years as parties opposed would find the only tool at hand - economic punishment - to be unsustainable. It would legitimise nationalistic sentiments in Europe even further. Britain would, naturally, talk of betrayal but not be able to make any resistance of any substance.

I think that would cause those who wield true military power to reassess the strategic landscape and dispense with the greater threat. Nahmean?

Judging from what happened in Ukraine, the King will be walking around in T-shirts a lot /s. In reality, only bad things can happen from this.

The US milirary will have hardcore maga cultists vs the "normal" conservatives + everyone else in the military that's pro-constitution and they'll fight each other. National Guards might also be join the fight. All Democratic governors will tell their national guard to disobey any trump orders and declare any of such orders unconstitutional, and they'll be ordered to help the pro-consitution side. republican governors... that'll depend. If they have a "Brad Raffensperger" type of governor, they'll also tell their National Guard the same thing and fight for the pro-consitution side, but then you have those maga republican governors that'll tell their national guard to help the maga side.

But it ultimately comes down to the people holding the guns and which orders they obey.

So.. to re-iterate: there will be an internal conflict before Canada would ever see the invading US forces, and if the pro-constitution side wins, they use the 13th amendment to remove any congress people that supported the illegal invasion, then new elections are held (and for the senate, temporary appointments will happen if the state allows it, and they serve until the special election). There would probably be a huge overhaul of the country, and it will be like post- Civil War USA, but this is v2. This will be the second "Reconstruction", hopefully it doesn't fail again like the last time.

I'd die. Thats what would happen.