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The areas were inherited by Mexico as the result of decrees made by Hernando Cortez who claimed nearly one third of what is now the United States for Spain without ever seeing any of it. There was no way to get there to see it as the desert acted as a barrier. The northern half of Mexico remains an uninhabited desert barrier today. What was eventually to become the United States was uninhabited except by a few crude villages with 15 people 60 chickens and two goats. --resembling a scene from a Clint Eastwood movie.
Also, Mexico was geopoliticaly unstable. It had been ruled by Spain, France, Germany and was a bargaining chip in many, purely European, deals. Mexico, as it was used by European, countries was a threat to American interests in this hemisphere according to the principles of the Monroe Doctrine. That there was a Mexico before 1910, is a specious argument.
elbucko!