It starts when Mexico was originally a Spanish territory. The United States government and the Spanish government encouraged pioneers to settle, in what is now current day Texas. After Mexico won its war of independence, it became uneasy with the vast amount of foreign settlers occupying their land.
Seems like Mexico had an issue from the very beginning with the US pioneers that were encouraged to move there by its previous “owners”.

Any discussion about it now is just academic and has no practical application because the only way Texas is going anywhere is if the people of Texas want to, and they don’t seem to want to very badly and if they did they definitely don’t want to be part of Mexico.