Truman confirmed the concept about civilian control over the military during Korea.
This is not the same sort of issue but does remind us that the President is the Commander-in-Chief of US Military forces and head of the Executive Branch of Government.
Just as Kennedy had to remind the Joint Chiefs of that during the Cuban Missile Crisis.
Just as Obama did in withdrawing troops from Iraq....against his military advisors wishes.
The Presidents are not always right....but they are always the President and it is in them the final authority lies.
If President Trump is trying to extricate us from a very precarious situation in particular and from the Middle East in general....combined with Energy Independence.....meaning less need to be in the Middle East.....perhaps that is good thing.
I do wish Mattis had stayed....but at some point these folks....the genuinely good ones do not stay too long for the very reasons they are the good ones.
Only Time will tell.