It's really a class prejudice as much as anything else. Mexicans are seen to be poorer...therefore lower class and presumably a threat in some way. A richer group of people always fear and resent an influx of poorer people. This has been the case not just in the USA, but in any society I can think of all through human history.

The Mexicans are resented and feared now just like the Irish immigrants were resented and feared by the general American public in the mid-1800's.

As for the oppressed situation of Native Americans in the USA, it sounds to me a lot like the oppressed situation of Tibetans under the heavy hand of China ever since Tibet was occupied shortly after the end of WWII...only that's a more recent conquest.

Injustice is not limited to the USA. Not by any means. It's found all over the place. (And I think we all know that.)