I have been reading about a “Reconquista” theory, which basically is Mexicans will legally and illegally populate the entire southwestern region of the United States. Once they reach a secure majority of the voting population they will vote to rejoin or become part of Mexico.
I doubt the rest of the United States would go along with this type of “Secession”. Consider a comparison to the US Civil War, the Union must be preserved.. The North forced it’s view upon the South despite a clear majority of the people living there being opposed. In that case the majority had lived there for several generations as may be the case (or not) with the US Southwest. It’s more likely the bulk Mexican ancestor population by the time it reaches a clear voting majority may only be 1 to 3 US generations old. While the minority within those states may be up to 14 US generations old .
There are a lot of very important military facilities within the region that if lost, would effect the country’s ability to defend itself from attack within. More so if any attack was backed by other governments, i.e.
Mexico supported by China.