US law (the National Security Act of 1947) requires that a military officer be seven years from active, uniformed service before he can be appointed to serve as Secretary of Defense. That law also provides that Congress can waive that requirement.
What is the waiver all about, Big Red Car?
That waiver has been granted twice in US history.
1. George Marshall — former Architect of Allied Victory in World War II, five star General of the Armies, Chief of Staff of the US Army, Ambassador (Special Envoy) to China, Secretary of State, President of the American Red Cross — would receive that waiver and go on to save Europe through the efforts of what came to be named the Marshall Plan.