Steal land from Mexico...are you historically ignorant??? You want to make a case against the US fighting Mexico for land back in the day....
The Spanish conquistadors had already done that and I'm figuring you are completely unaware of how much worse they were than the British colonization of America and the future efforts that resulted in the creation of the United States.
Your history tellers might give a candy coated version that the Spaniards made an attempt to assimilate the indigenous population but the reality of what they did was nothing short of horrific.
The Spaniards killed anyone that resisted and after achieving dominance they proceeded to traumatize the population by making a point of cutting off the legs of anybody including children that tried to run away in front of everyone, they also poured boiling soap down their throats of others while again making people watch,less fortunate people were hung from a rope on their tip toes and light on fire while they took bets on who would last longer, they went on to rape the women/female children and that is just the short story....then gave the rest of the population a choice assimilate their culture or basically die...that is just a drop in the barrel of what the Spaniards did to the indigenous population of what is now called Mexico.
At least in the United States we still have reservations for the natives of this land and give them money every year.... question how many reservations are in Mexico and how much money do the indigenous tribes get in that country from their government???