“I cannot help but ask a question — what if in the future, the vast majority of a California city decide to join Mexico? Will the USA let it?”
That is exactly the issue, and the reason diversity and the failure to assimilate is such a bad idea. Liberals, with their insistence on keeping groups separate and identifiable have set us up for real problems.
Look up “The Conch Rebellion.” Key West succeeded from the union and declared independence. The US government set up a customs station and harried the heck out of people going both ways as if it were a border, making it a several hour affair in the hot sun. They also cut off power and water until Key West decided they had made a mistake.
I was astonished at how seriously the government took that bunch of bozos and at the hardball the government played.
Well yeah if you want to be not part of Florida then you got to live with it. That’s survivable if you are a heavy commerce center like Singapore. Not for kicked back Key West.
Problem too with Caleeeforneeea that isn’t often mentioned is that its many cities and towns were founded with spanish names. Los Angeles, Santa Cruz, San Rafael etc...
so the south of the border types have a natural affinity to be associated with it. Of course there are other factors but this is a factor IMO —albeit not the major one for illegals.