I posted the history of the State of Arizona as posted on a State of Arizona website.
It belonged to Mexico, now all you guys are busy excusing off people taking land that doesn't belong to them.
It belonged to Spain --- and then it belonged to Mexico a very short time. Spain never was able to settle the northern territories very successfully because it could not convince the Mexicans to move to the areas where the Indians of what is now the SW USA would attack and kill them. The northern territories were very sparsely populated up until recent times --- well after they belonged to the USA.
And Spain stole the land from the Indians. Try putting a Don Onate statue somewhere in the SW USA --- and you'll quickly find out the problems the Indians and even Chicanos had with Spanish rule of these territories. And amazingly --- the descendents of the Spaniards in the SW --- many Spanish-American Indian descendents have never objected to becoming part of the USA. In fact Tejanos helped fight off the Mexicans in the war of Texas independence. And what might be more amazing to you yet --- if Mexico tries to annex the SW --- you will see Spanish named people gladly going to war against Mexico. You have no clue how it is here --- long time Spanish/Indian Americans of the SW do not want to be part of Mexico, most are very patriotic Americans --- and many are opposed to massive immigration from Mexico for the same reasons as everyone else.