Is there a legal procedure at all for Canada to become the 51st+ state of the USA?

AFAIK, there was a period of negotiation between West Germany and the last, democratically-elected, government of East Germany - but since both governments were dominated by Christian Democrats and the East had clearly voted for incorporation within existing constitution of the West, the Western plan went through quickly. (It involved reorganising local government in the East into five new Länder, but that doesn’t seem to have been a big issue).

Oh, and anything to do with the Commonwealth would be a dead letter in this situation. It would be up to all the member states, but I can’t see them voting to admit a USA that’s just strongarmed Canada into anything. Nor, I assume, would the USA want to assume responsibility for a commensurate share of funding for Commonwealth projects. Plus, as the newest member, it would come last in ceremonial orders of precedence.

Canada (and the UK and Australia and most of the rest of the countries Chuck is king of) already has a republican form of government, despite having a monarch.

Has someone already coined “Canschluss”?

No, because although King Charles has no real power over Canada, the Governor-General officially represents the monarchy in Canada, and as Head of State the GG actually does have considerable reserve powers, though the discretionary powers are rarely exercised. It’s definitely not a republic when the GG, acting on behalf of the monarch, has to give royal assent to every bill passed by Parliament in order for it to become law.

Like wolfpup, I think I would have to disagree with this assertion as well.

Wyoming and Rhode Island might be tired of being least populous and smallest states so they’d go for it.

You contradicted yourself, I suspect because you are confusing the concept of a republic with the concept of a representative democracy.

Technically, this is true. But first there has to be sixty votes in the U.S. Senate to stop debate. It is almost impossible for one U.S. party to control that many seats. Since adding a new state or states would advantage one party or the other, and the other party would vote against, it is impossible for the foreseeable future.

It was used pretty early in the original “trump wants to invade Canada” thread.

Does it matter? I think in this event, the Canadian constitution no longer has any force of law.

I highly doubt one country can unilaterally steal another country’s territory like that.

Quite true, but I think besides that, there are many other reasons that admitting Canada as one or more states would encounter great opposition in Congress. And then on the other side, there’s no way that Canada could agree without a national referendum, and I guarantee that opposition in Canada would be almost universal. There are already big anti-US boycotts just because of the threatened tariffs, plus the sociopolitical culture here is completely different in many important respects despite superficial similarities.

So, IOW, ain’t gonna happen, because neither side wants it, despite Trump’s uninformed fantasies. There’s also the fact that the whole Canadian system of governance is structured around complicated federal-provincial divisions of power. Canada is not just one place that could suddenly become “a state” – it’s much more complicated than that. The whole idea of any kind of merger is appallingly ignorant, aside from the fact that no one on either side wants it.

Isn’t “representative democracy” the definition of “republic”?

That’s my thought as well. If, hypothetically, there was an “unconstitutional” referendum endorsed by a a super-majority of Canadians to join the US under a new (Canadian or series of Canadian) proto-state constitution(s), and the nay-voters weren’t outright going to war to prevent it, I should think the Canadian constitution would be quite irrelevant if and when the US were to pass appropriate legislation on its own end.

Because apart from the would-be state needing to come up with a new constitution and apply for statehood, the US Constitution appears silent on any niceties of Canadian domestic law.

No, not at all. Many representative democracies are not republics, and a republic does not have to be a democracy. I realize Americans have gotten into the habit of using “republic” as meaning sort of the same thing as democracy - after a few centuries, it doesn’t make much difference in the USA - but that’s not really what it means in a technical sense.
China’s a republic but it’s not really a democracy, though its governing body purports to represent the people and, in a technical sense I guess, does. A republic could be a democracy but not a representative one (I can think of no existing examples; I guess ancient Athens was one. It’s impractical.)

A republic has no power even theoretically higher than the people. In effect the government is the collective property of the population at large. In a monarchy like Canada, the government governs on behalf of the monarch. They may be elected - in fact in the British system they HAVE to be, that matter was settled centuries ago by lopping off a king’s head - but the government still has a monarchical head of state not themselves subject to politics or elections. The government BELONGS to the monarch, and in a constitutional monarchy, the elected representatives make the political decisions while the monarch reigns as a symbol of the state. (Of course some monarchies like Saudi Arabia are essentially absolute monarchies, not at all democratic.)

I know it seems like a distinction that’s just for show. It’s not. It’s actually a very fundamental part of how our system works. Investing a nonpolitical person as the personification of the state away from politics is a very deliberate element of the Constitution.

You’ll notice that in @Little_Nemo’s formulation, Wales wants to change which country it’s part of. No stealing.

Of course the process of Wales getting to “wanting” and Wales’ current government figuring out how they would assess the degree of want is pure smoke and mirrors here.

How do you think Wales became part of the United Kingdom in the first place?

This was exactly how Russia took Crimea, by force. They then relocated people to Crimea to force a majority vote for annexation.

I did notice, but I suspect the UK government would have some say. Or are you saying that if Mexico passed a law annexing California and California wants to be a part of Mexico that it is a fait accompli and there’s nothing the US government can do about it?

Which is not what Little Nemo said. In his analogy Russia passes a law annexing the Crimea and Ukraine has no legal say in it and MUST turn it over. If you think he’s right, then you’re saying the US can pass a law annexing Greenland (assume the want to be part of the US) and Denmark can’t do a thing about it. Is that what you believe?