Um, there were disputes in the area north of the Rio Grande during the early to mid 1800s. The Mexican government wasn’t doing anything to protect Spanish, now Mexican colonists from Indians raids, mainly by Apaches.
Mexico had a highly centralized government and if an army officer wanted to be promoted, it wasn’t going to happen away from Mexico City in some dump of a fort up near the Colorado River! Much of Mexico’s problems as an independent nation extend from the fact it maintained a colonial-type centralized government for way too long.
Yankees could get their government to come out and protect settlers. Former Spanish colonists noticed. Not everyone had loyalty to Mexico.
Just as an aside, my paternal grandmother’s parents were Welsh and my great grandfather was a collier, a coal miner. One place he worked as a miner was a gold mine in northern Mexico.
Pancho Villa raided the mining camp and forced everyone - men, women, and children - into the mine, then stacked explosives up against the mine entrance. The Mexicans were driven away by the U.S. cavalry before they could blow the entrance and bury everyone alive.
Needless to say, my great grandparents couldn’t move fast enough to get back to the sanity of the British Empire in Canada!
This period of time before the Depression was very volatile in Mexico’s history. There were frequent raids back and forth across the U.S.-Mexico border.
Just recently went through what is known of Spanish settlement in New Mexico (a far larger area than at present) in 1598.
Did you know the Santa Fe Trail, which starts, of course, kinda near Santa Fe actually ends at Dan Boone's land grant at Arrow Rock?
There are a couple of stones in the vicinity that look like early pre-1598 Spanish base-line markers (you lay a baseline in before you do surveys).
We have similar lines in the East ~ one of them seems to parallel today's US 50 East and West, then there's one along the old Carolina Road (US 15), and a series of markers along the top of the Eastern cordillera from South Carolina to New York . The best line starts at Arcola on the Gulf of Mexico and runs due North to the Kensington Stone. I think the particular bunch of Spaniards laying in that line were Swedish (BTW, I know of a number of other points of reference of great significance along that line but I'll not let folks know about them until i write the book.)