From: ENCYCLOPÆDIA BRITANNICA
"Despite the acceptance by many Americans in the 1840s of the concept of Manifest Destiny—that it was the providential right of the United States to expand to the Pacific Ocean—the future boundary between the United States and Mexico was anything but a foregone conclusion."
that's why in the Southern states one finds an extraordinary number of streets and cities with Spanish names.
and that's why many Mexicans hate the Gringos
this post written in Ciudad de Los Angeles