"They could have just gone back home except that they were making too much money. Texas wasn't their country."
Rather glib, and overly simplistic, but true enough. 
Riddle me this then.  Name anyone except the First Nations that should be here?. 
Land is taken by force.  Be it economic or physical, the end result is that one side loses.  Denying that is denying history itself.  It seems you forget that the land didn't belong to the Spanish / Mexicans either.  The spaniards enslaved and slaughtered the aboriginals without much forethought.  The land also didn't belong to the Aztecs in Mexico.  They took it by force and bloodshed, using slave labor to build temples and cities.
My point is, the entire history of humankind is built on the corpses of our own stupidity.  Whether you teach about it or not, the end result is the same...you are where you are because of what happened.