As in were the people that lived considered natural born citizens, green card individuals, or just plain illegal residents?
So this is an old question but I found it while searching so I though I might answer!
The Hispanics who lived in New Mexico were Nuevomexicanos, and weren't like other Mexicans. They were a mix of the Spanish settlers and the local Pueblo peoples, not Aztec and Spanish. So they had little in common with the ruling class of Mexico. Nuevomexicanos were the losers when Mexico gained its independence as they were still loyal to Spain. So New Mexico would have rebellions against Mexico every now and then that had to be suppressed. They did however manage to successfully kill the Mexican appointed 10th governor of New Mexico during the Chimayó Rebellion.
America saw they had no loyalty to Mexico so offered them citizenship and promised them retention of their land grants if they helped fight with the USA and against Mexico, which they did. For their loyalty to the USA they got citizenship. Because at the time white citizens could only vote, Hispanics were considered white, something still practiced today.
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