Mattis may have scrubbed the DoD of the DS as well as part of his role. A lot of that may have been about eliminating cabal foreign (Chinese especially) contracts, who knows?
The public side was managing rearmament of the military which POTUS says is now the most powerful in the world again US steel and electronics?).
Personally, I think Mattis is a patriot who loves the military above all else and probably delayed his retirement for this role after seeing so many generals given the axe by Renegade and what that traitor did to weaken/destroy our defenses.