Most of the area your map calls 'Mexican Republic' was Spanish-claimed territory, as no other world power had claimed it yet. In 1769, Spain advanced
north of the California Peninsula in part because it was feared Russia or England (pre-U.S.) would claim that land.
Mexico achieved independence from Spain in 1821 and California Spanish officials learned of and accepted Mexico's victory in 1822. So Mexico
automatically got the Californias simply because they were Spanish territory.
Mexico (nor Spain before) had the population or ability to very well occupy, protect, or defend the vast territory northwest of Mexico City.
Obviously, the United States proved itself far more able to occupy and develop the land. At the end of the short Mexican War, our troops had occupied
all of Mexico and were at the capital. In essence, all of Mexico could have become part of the United States. Not desirous of such a new burden,
America wisely limited its acquisition to Alta California, which it did pay Mexico for. Mexico has benefitted ever since by have such a wealthy
neighbor. |