Over at War on the Rocks, I have a post explaining that a new statute will be needed in order for General Mattis to serve as Secretary of Defense. It's been done once before, with George Marshall during the Truman Administration. Read the post to see how Congress on that occasion carved out an exception to the National Security Act rule forbidding a retired officer from becoming SecDef for a certain number of years (it was 10 years then, 7 years now), but in doing so purported to bind itself not to do the same thing again in the future. Needless to say, that attempt does not control Congress today. I included the full text of the original statute, which may well be adopted soon to pave the way for a Secretary of Defense Mattis.