I think you are a tad bit off... It's 60 feet. It is also only from the Pacific to the New Mexico/Texas border. Texans own the land to the border, and will take care if things themselves anyway like they have been for a couple of years now - as the wall is finished.

For those that don't know - here is a decent wiki article on this: https://en.wikipedia.org/wiki/Roosevelt_Reservation

What it means is the feds (the Dept of Interior actually) own a 60' swath of land along the southern border everywhere but Texas. Teddy Roosevelt did that to stop border smuggling. The DOI just transferred it to the military. Now Trump is making that very long 60 foot wide piece of land a US military base. Why? Well, Posse Comitatus says the military can't enforce civilian law without special situations (usually for the National Guard) or if they are deputized.

Ok - but still - why? Easy. By making the Roosevelt Reservation a military base, the military can protect that base - including the use of lethal force. They can always act as law enforcement on their bases - it is federal property. If illegals, smugglers, cartels, invaders, etc. enter that area the military can stop them. They can defend the base - including when the cartel shoots at them. We will shoot back.

The border is now closed.