After the American Revolution, for 50-years the Mexicans attacked Americans in the Gulf of Mexico, murdered them and stole their cargoes.
Our young government did nothing about it, mainly because the Mexicans had thrown out the highly centralized Spanish Colonial government and replaced it with the highly centralized home-grown version, that Washington, D.C., figured would correct itself over time. That was a mistake.
It was during that period that Mexicans swore, that no matter what happened, they would never give up seizing back THEIR territory that the gringos had stolen.
I never forget, no matter how nice a Mexican person is when introduced, that they are a potential enemy.
I read, speak and write Spanish fluently and know the culture inside and out. Americans have no idea how much of an enemy Mexicans are. I tried to, in a more indirect way, tell them that in school and enraged everyone. You can NOT help people who don't want to be helped. Also, found the statement by Will Rogers which is so true: "It's not so much what you don't know that will hurt you. It's what you know that ain't so." In other words, what so many folks believe about religious issues, other people, women, men, children, etc. simply ain't so and they'll pay for it.