The nomination of recently-retired General James Mattis to be Defense Secretary will require a waiver of United States law regarding civilian control of the military. 

Civilian control of the military is a defining element of modern democracy. It was established in our Constitution and continued in law with the creation of the Defense Department in 1947. In recognition of its importance in the world, it is a requirement for membership in NATO. The law forbids active or recently retired armed forces members from being Defense Secretary. Most countries without this are some form of dictatorship.

In the nearly 80 years since, only one waiver has been granted, in 1950, by Congress for George Marshall, for one year only  to help the build-up to the Korean War. While Marshal had been a general, he also had significant post-military civilian government experience, having been Secretary of State.

This is not about James Mattis the man in any way; it is about a critical Constitutional principle that has helped define America in history.