#1Mar 16th, 2017
For something like the US becoming Canada's 11th province would require some very extraordinary changes in the US, Canada, and the relationship between the two.
Which opens the door for me to speculate about THAT! (Answer the question you want to, not the one you're asked.)
Currently, the population of the US is about 9 times that of Canada.
Let's assume that that this was a voluntary union. (I.e.: playing up on the whole "Canadians are very polite people" thing.) What would cause the population of the US to abandon our existing government structures (peacefully) and wish to join Canada? What would convince the Canadians that inviting the whole of the US in as an 11th province was a good idea?
A massive de-population of the US is a horrible thing to contemplate - unless they (we) all went off some place else voluntarily (and where would we go? Australia?) However, a massive depopulation of the US would make it easier for Canada to bring in the US as a province. In order to do that on equitable terms with Canada's existing provinces, the US would have to loose 99% of it's population. (Let's assume the Australians don't object to a big American ghetto in the middle of the Outback - although where the water would come from is quite a puzzle.)
An alternative possibility is that something caused 90% of the US population to flee to Canada - and Canada decided that the best solution was just to take over the states and put Those People (us) on a bus back south, after taking over the reins of government in the US. That's almost plausible - the Canadians have generally been welcoming of refugees, but dealing with a huge influx of people from the US would overwhelm them and simultaneously leave a large part of the US empty right on their doorstep.
It's hard to imagine something that the Canadians would offer that would cause the US population to turn over the existing constitution and petition to join Canada without some kind of disaster. Canadians just aren't perverse enough to offer something that'd appeal that broadly. So far, the best they've done is Justin Beiber - and the only knowledge I have about him is second hand, so I can't attest to the true perversity there.
The Mason-Dixon line would become the border of the Confederate States of America (CSA) once again, but the line would alter to include parts of the American northwest, especially Wyoming, Montana, Idaho.
Washington east of the Cascades would vote to stay in the CSA, while the Washington peninsula area would join with British Columbia. Vancouver WA would have to change its name because there is already a Vancouver, BC and the Canadian Postal Service would never be able to get mail to the right Vancouver.
Hawaii would go independent again, but might join Canada voluntarily if enough Canadian dollars were invested there because after all, Canadians want to have some place warm to go without paying the ghastly (to them) CDN/CSA exchange rate.
Canada would put up a wall along the border to the CSA and insist on making the CSA pay for it.
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