-The United States didn't take the territories you mention from the people residing in them - it took them from the country of Mexico, who at the time really wasn't doing much for the people within them, except for taxing them heavily while offering no protections or benefits in return. Which is why most of them didn't object to what happened. -The majority of the Spanish/Mexican population within those territories welcomed their acquisition by the United States because of Mexico's distraction from their prosperity and well being. So much so, that those in Texas had joined in a war of independence from Mexico, and the establishment of the Republic of Texas(more "Mexicans" died defending the Alamo than did Anglos). -The indigenous Spanish/Mexican population in these territories were not forced from their ranch land and farms, and their orchards, crops, and livestock were not destroyed. -Children of the indigenous Spanish/Mexican population were not regularly killed and maimed because of the use of an inequitable and deadly response to protests - because there were no protests to speak of, because of what's mentioned above. -And finally, the people residing in these territories acquired the same rights, priveleges, and protections as every other citizen of the United States, and have been completely assimilated into American society.
from the article: Israeli troops shoot dead Palestinian boy in West Bank