I don't think there is any question that there are military and LEO's in this United States that would follow orders to disarm and even to kill citizens in such a horrible scenario as you bring up.
Not all would, but there are some, maybe more than we would believe. It has happened before with civil unrest and protest.
What I question is the ability and commitment of the American citizens to actually resist such a gun grab.
Would you, without question, fire upon military or police personnel who were asking or ordering for your guns to be surrendered?