Historianon here-
We paid for this land, all of it. Saved Mexico from being overrun by Britain and Spain for loans they made to Santa Ana. France wanted land and not repayment, hence empire of New France which we sent 50,000 armed troops across the border (Gen Sheridan, Civil War) to help them purge the French!
Texas fought for their independence from Santa Ana after he slaughtered 1000s in Zacatecas state (one of 8 in full rebellion) the year before. They were in uprising because Santa Ana suspended the Constitution of 1824 and made himself dictator for life.
This land belongs to the US. Don't forget Anons! Rant over. <mic drop>