There were some, though not a great many, people of Spanish extraction living in the area the US bought in the treaty of Guadalupe-Hidalgo the area was North of any area that the government in Mexico City made any attempt to territorially control. For the most part, they considered themselves to be Spanish and were antagaonistic to Mexican government before the Texas War for Independence.
Memebers of the US Army that participated in the Vera Cruz-Mexico City campaign opposed annexing all of Mexico, causing congress to buy (at the point of a sword) the part of Mexico that "didn't have any Mexicans living in it."
Under the terms of the treaty, they had to declare their intentions regarding citizenship within one year of it's ratification or they would automatically become US citizens as a result of living in annexed territory.
There were more peole indian tribes with their various languages in that area. Except for the Comanche, they seemed to get along with Americans, mostly. The troubles didn't really start until some racists in Colorado that were members of the territorial guard and wore union uniforms started attacking indian villages.
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