I don't pretend to fully understand this but the Secretary of Defense and his department are one of those ordered to cooperate with DHS in providing defense of the nation.
Seems to me that THIS DIRECTIVE D5 allows the use of the Military Forces domestically if they are supporting DHS. It might be that ICE and HSI need to be the front line folks pulling the triggers of weapons, but there are DOZENS of support roles that the military could fulfill which DO NOT INVOLVE USE OF FORCE AGAINST CIVILIANS.
Nobody seems to have noticed this. The effect of this would be like an AVALANCHE if POTUS really does have a lawful constitutional means of deploying army, navy, air force, marines on American soil to be used against American citizens involved in subversive activities.
It would mean that any plan the left has to concentrate the use of force in one geographic area to OVERWHELM law enforcement, simply will not work because there is plenty of military to back up any and all law enforcement.