In order to prevent the military from seizing control of the country, the US decided that it must always have civilian oversight of the military. It is a law that only a civilian can be the secretary of defense.

Perhaps I should have been more clear. I realize how difficult is for you to understand things. What I meant is that IMHO we need someone with military EXPERIENCE in that position.

Mattis will be our next secretary of state as congress will give him a (legal) waiver. Anyway, he IS a civilian - he retired from the military in 2013....