United States unamendable national and state borders are also a factor. Whatever one thinks about Brexit, it allowed EU to continue with countries that are committed to EU and for UK to consider what's best for UK, including welcoming Hong Kong residents not enthusiastic about takeover by China. If California and Texas go their own way and borders between states are adjusted to accommodate red and blue area, maybe cu…
I've spent the last past decade pointing that out to people. People act like a state being separate from the US and independent is soooo absurd, logistically.
But the US can still assist on a natural disaster just like it does in all of the Americas. Common defense can occur, as you point out. Visa and waivers could be easy.
I always like to imagine what level of state tax would I tolerate if I was only paying income taxes to just California. Since California doesn't get much/anything back from the Federal Government from its remittances, I bet California wouldn't even need to raise its states taxes.