The use of lethal force is always available for self-defense, whether armed or not.The President doesnu2019t need to u201cauthorizeu201d it, it exists by law, regulation, and civil rightu2026Self-defense includes the defense of others, so an armed, or unarmed, soldier at the border who witnesses an attack on anyone, anyone at all, is duty-bound to come to assistance (or perform another action such as get help or act as a witness) of the victim.If an armed soldier witnessed someone shooting or beating another soldier, or a civilian, or a Federal officer or employee, then based on the circumstances, that soldier has to make a decision as to whether lethal force is authorized.Such decisions are built into the rules of engagement provided to soldiers during any deployment such as this, as well as military and civil law and regulations.(EDIT: ROEu2019S APPLY OUTSIDE THE US, INSIDE THE US THERE ARE ALWAYS STANDING RULES FOR USE OF FORCE [RUF], THAT ARE MODIFIED BY THE COMBATANT COMMANDER [NORTHCOM INSIDE CONUS] AND THE SECDEF AS NECESSARY. SEE: https://fas.org/irp/doddir/dod/d... and http://www.jag.navy.mil/distrib/...)