There is a long standing  principle in the U.S. that the Secretary of Defense should be a civilian so that there is civilian oversight of the military. Hence the rule that no former military officer should be Secretary of Defense until he/she has been retired from military service for at least 7 years. The only prior exception was George Marshall, who was appointed in 1950, 5 years after stopping down as Chief of Staff of the U.S. Army. This was a one year appointment where he was tasked to help the Army, which had been greatly decreased in size after 1945, recover to fight the Korean War.  In between he had served as Secretary of State for three years, establishing the Marshall plan to resurrect Western Europe  after the ravages of the war and received the 1953 Nobel Peace Prize for this. Mattis is clearly  different since he has kept  military ties in his 3 years since retirement. If he tries to use Marshall  as a precident, the response should be, 'Mattis, you are no Marshall.