Texas was a sovereign and independent country that freely joined the United States.
So there is NOT anything to "Give Back".
Texans fought, bled and died to wrest freedom for them and their posterity from a hostile government and won. The former rulers officially signed over rights to governance to the new free Republic of Texas.
Texas then as an officially, legally and solidly recognized independent country, decided to merge with the United States and become a State.
So all this B.S. ideas about "giving Texas back or such" is a bunch of revisionist B.S.
Now California...
They should have never stopped conquering until they took the whole peninsula, then we wouldn't have half the troubles there we do now.
Perhaps in the future that will be done as payback for all the crime, drugs and illegals that the Mexicans have pushed into our country.