The Mexican is reconquering the Southwest through invasion and birth rate. They lost it in the 1840’s
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The republic of Mexico surrendered their paper claims to the US in 1848. In point of fact Mexico and Mexicans had almost no hold on the American Southwest outside of a few mission towns. Most of the land was occupied by tribes who were hostile to Mexico, spoke neither Spanish nor Nahuatl and who to this day take great offense to being categorized as ‘Mexican’ or lumped into the fantasy nation of Aztlán.
In California the locals had already seized the Mexican garrisons before the American military arrived.