As for distance to engagement, its relative to what you are trying to accomplish.
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If Mexico is unable to stop them, use gentle persuasion to allow us to overfly their airspace and have the Air Force run air strike exercises over the horde. Run Bones at 50 feet on full afterburner, I am sure it would get their attention.
For a division of infantry ‘gentle persuasion’ means depleted uranium ... or M142 High Mobility Artillery Rocket System (HIMARS) (included in Warning Alert issued by Pentagon).
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