What we may be seeing here is, though someone may be a great general, that person may be naive as to politics and the damage those with significant ideological views can have on a government department. That appears to be a significant weakness of Mattis’.
Nobody naive about politics gets 4 stars in the military.
Allow me to direct everyone's attention to the example of Colin Powell.
I get your point, but they know the politics of getting promoted, not necessarily the real gut politics of ideology and party. As a star officer, he expects people under him to follow orders and support his positions. If not, they are removed. I expect Mattis thinks that is the way it is in the political world — ain’t so. Don’t get me wrong, I’m not excusing Mattis’ personnel actions — they are very disconcerting.