Yes, Trump needs a waiver to appoint Mattis as Defense secretary

Unlike Shannen, though, I do not believe Congress’s power to legislate qualifications for an office it did not have to create in the first place is limited to the incompatibility clause (art. I, sec. 6, cl. 2 – which, as Shannen observes, prohibits members of one branch from serving in another).

For instance, I am confident Congress could require that the Secretary of Defense must be an American citizen, over the age of 21, or of sound mind. (Compare, Section 504 of Title 10, U.S. Code, governing qualifications for service in U.S. armed forces.) Such conditions would reflect the concerns of the Framers, who worried about the possibility that the central government’s most awesome powers could fall under foreign influence or into incompetent hands. Similarly, I see no constitutional problem with Congress’s mandating that, to qualify as Defense Secretary, a nominee must come from civilian life or be removed from military service for a certain number of years. Such conditions also reflect the Framers’ qualms about national armed forces and the need for strict civilian control over them.

The strongest argument in favor of the proposition that the Constitution prohibits congressional limits on the president’s choice of a defense secretary is the 1789 act that established the War Department and its Secretary. (See here, scroll to pp. 49-50.) This statute empowered the president to appoint anyone of the president’s choosing. There was no restriction requiring that the secretary be a civilian, much less removed from military service for a period of years.

But does the fact that the first Congress declined to prescribe qualifications signal its understanding that the recently ratified Constitution forbade it from doing so? Or did lawmakers simply decide not to prescribe qualifications because they trusted the judgment of George Washington, and were aware that he would appoint Henry Knox, a hero of the Revolutionary War and close aide of Washington’s who, for several years, had been serving as secretary of war under the Continental Congress?