I am reminded of a somewhat similar and less-fraught exchange of populations between the US and Canada. The United Empire Loyalists left the US for Canada after the American Revolution and helped spur the development of Canada. Over the years, Canada and the US have been on mainly friendly terms (except for that "contretemps" in 1812). Also, there has been a slow "brain drain" or "talent drain" from Canada to the US that has been going on for well over a century at this point.

My immigration lawyer in the 1980s wrote the textbook used in law schools about US immigration law. According to him, about one third of Canadians could get a US passport on demand, and well over half of all Canadians could get a US green card on demand, at that time.

I am not so sure this remains true currently, because in the last decade Canada has remade itself in a way so as to be almost unrecognizable, in many respects. If one trusts surveys and polls, the vast majority of Canadians are not very happy about this.