The original con job.
The land never "belonged" to Spain in any real sense; it was only printing on bad maps and the wishful thinking of a self-impressed, but historically irrelevant society -- the Spaniard Conquistdors and their descendants. The pitiful attempt at colonization by baptism of the Indians in California and sustained, but numerically small settlements north of the Rio Brazos, merely points out the irrelevancy of their 3 centuries of "ownership" of the land that is now referred to as the Southwest.
Spain did not "own" the land north to the pole, even though the Pope may have declared it so; Mexico could not "inherit" what Nueva Espana never had; and the United States did the right thing in 1846 by securing both coasts against monarchial domination by any European power. History has proven that this was one of the best things to have ever happened.
The present mass exodus from the glaring failure country of the Americas threatens to end the shining achievement of the greatest success story in history, the country known as the United States.
Some of the land in question was territory of Spain -- the Spanish land grants are still recognized --- but that wasn't the Mexican government --- if Mexicans think that land that was considered theirs for about 25 years must be returned, then all of Mexico would have to be returned to Spain.