The Mexicans are part and parcel with this. Ordinarily they dont permit invasions from Central American nations.
This would be considered an act of war. Last time we went to war with Mexico we took Texas New Mexico Arizona California etc.
So Mexico you have to ask yourself. Do you wish to lose millions more square miles of your nation?
Mexico let them in, make them Mexico’s problem.
They must be denied entry to the US.
They don't when they think those invaders plan to squat in Mexico. When those invaders are en route to the US, it's a different story, because then they become our problem, not Mexico's.
I believe it was the new Republic of Texas that got all that land. Right?