Yep, the Spaniards stole it from the Indians, and we stole it from the Mexicans, (as well as stealing plenty from our own Indians). All humans, but Europeans in general, and Anglo Saxons in particular, have a splendid record of racism and imperialism. Of course, they always justify it with the best of reasons; like Manifest Destiny, bringing Christianity and civilization to the savages, making sure good farmland doesn't go to waste, "progress", etc.

In Mexico in the 1500s, as it was in California in 1849, the real reason was gold. In Texas, it was the god-given inalienable right of Anglo Texans to establish slavery, even if it was against Mexican law. Of course, Anglo histories tend to carefully avoid that interpretation.

Sure the reasons are genuinely complex, and rarely one-sided, but if you really want to know what the historical roots of the Mexican Problem are, look at the huge red piece of North America in your little map that represents the land Mexico was forced at gunpoint to cede after being invaded by the USA. It would probably be justice if the aboriginal inhabitants of that land were the ones to inherit it, but they have been rubbed out, by cultural if not actual genocide.

Unfortunately, in history, those forced to pay restitution are not always those who benefited from the crime.
Like I said, paybacks are a bitch.