Current day Mexico is not the Spanish empire at the height of it's power and reach. Spain failed in the geopolitical game against the Anglo-American powers.
Every country in history has been established at one point or another by conquest. If Mexico & Central American continues to threaten the internal security of the USA by subversive and belligerent invasion, than they will have to re-learn why their ancestors lost the territory that they did.
If they are all coming here for a better life it's because they recognize the superiority of the way of life and the oppurtunities in America. If we annex those failed states to our south, and implement our rule of Law, then the threat they pose will be neutralized.