[Southack]

"...patrolled by the National Guard which would make good training for rotation to the Sand Box. That fence can then be backed up by electronic surveillance and UAVs in the air. The combination of those things in depth..."

Just a nitpick, but redundancy or double/triple checks on a front line do not qualify as a defense in depth. A rearward fence or moat...an auxilary mobile force in the rear...that would qualify as defense in depth.

[Jeff Head]

Electronics monitoring approach to fence...fence...road...fence...electronics monitoring 100 yards inside US border...and UAVs overhead would be a barrier in depth in my view.