So if I get your reasoning right it wouldn't be a big deal if the U.S. seized NFLD, NB, NS, PEI, and forced a farce referendum to join the States, because Canada is irrelevant in the grand scheme of things?
Do you honestly think Ukraine and their citizens are irrelevant?
OK, let's go with that. Let's say the US seized NFLD, NB, NS, PEI, and forced a rush referendum to join the states, what would we do about it?
Now the US would claim that the majority voted to stay in the US so we must respect democracy, while the new alliance against the USA would have doubts about that. So what would be our options?
One option I guess would be to insist on an internationally-monitored referendum take 2 and the combined alliance could impose strict sanctions until then. If the US claim is true, then the US might not mind that as a way to end this standoff and rebuild its economy.
Another option could be to simply insist that the US return those provinces to us regardless of the people's will to remain with the US. That could be politically difficult. Even if the majority wanted to remain with the US, the US would still have been wrong to invade and Canada would still have a right to insist on getting its land back. In practical terms though, just compare the US' military might to Canada's and tell me what we could do about that. If we can't fight militarily, then we might be able to fight a battle of wits. If the US claims that it won a fair referendum, then it should not mind calling another better-monitored referendum. Should it refuse, we could then legitimately challenge its moral claim and even Americans would support us on at least that. It would be more than reasonable a request seeing the US had no business attacking Canada in the first place. Should the US call the referendum, it's properly monitored, and the provinces vote to stay with the US in this hypothetical scenario, given how the US had no business annexing them in the first place, the alliance might still be able to pressure the US through painful sanctions to at least agree to recognize any Canadian citizen born after the annexation date as having the full freedom to work there visa-free. Again, seeing how the US had no business attacking Canada in the first place, that would be more than reasonable and the US could maybe accept that.
The other option would be for the alliance to just maintain sanctions on the US until the US unconditionally turn the provinces over without a referendum. The the US refuse, then what? We just languish for the next century?
The members of the alliance could perhaps help Canada's languishing economy in such a scenario through preferential trade and visa policies. In practical terms though, what more could they do?