First we have to undo the treaty we signed with Mexico promising not to patrol our borders with troops. It scares them.
We have a treaty with them not to patrol the border? What room-temperature IQ idiot stuck us with that?
Darn straight we need to undo the treaty. Just scrap it unilaterally and let Mexico squirm. What are they going to do, invade us? Oh wait, aren't they doing that anyway?
A history buff will have to refresh my memory but we signed a treaty in the 19th century or early 20th that we would not use troops to patrol our southern border because they were scared we would invade them. I think it was an off-shoot of the Spanish-American War. They saw what we could do in Cuba and it made them nervous.
The Border Patrol was instantiated as result.