No different than the US and England.
The weird part is that for Mexico to have valid claim to the SW, you have to accept several incompatible POVs.
The Spanish claim to the area was based on utterly Eurocentric legalisms of who “discovered” the area. Which I though leftists objected to, claiming that Columbus didn’t discover anything, as people were already living there.
The Mexican claim is based on the right to revolution, but still on the notion that the Spanish claim was valid, just taken over during their revolution by the Mexicans.
A little-known fact is that Mexican independence was won not in the name of freedom and human rights, but in the name of continued special privileges for clergy, nobility and army officers. IOW, the exact opposite of American independence.
Which means for Spanish claims and the descended Mexican claims to be valid, you have to ignore the claims of people who’d actually lived their for millenia. Most Mexicans have some Indian blood, but it’s almost always from tribes who lived southof the present border.