President-elect Donald Trump may be expecting opposition from Democrats in Congress when it comes time to approve his nominations for cabinet positions. But what about opposition from a decades-old federal law?

In the case of his presumptive choice for Secretary of Defense—retired Marine Corps General James "Mad Dog" Mattis—that may be exactly what Trump faces. That's because a little-known 1947 statute requires that members of the military must be retired for at least seven years before they are eligible to become the Secretary of Defense.

Mattis retired from the military only in 2013, so Trump will need a special waiver from Congress if he wants Mattis as his Defense Secretary.

"There's a policy that essentially says, 'we think it's better if a military person does not run the Pentagon,'" explained Juliette Kayyem, homeland security expert and host of "The SCIF" podcast. "Why? Because when you have someone who has risen through the civilian ranks, they can look with a wary eye to what the Pentagon is telling them or wants them to believe."

"Picking these guys who are just out of the military doesn't give you the global perspective you would want from someone who is Secretary of Defense, who has to work with states, the CIA, the Department of Justice," Kayyem said.

There have been exceptions to the rule. General George Marshall—who led American troops during World War Two and led America's plan for reconstructing post-war Europe as Secretary of State—became Defense Secretary under President Harry Truman in 1950. Truman sought and received a waiver from Congress for Marshall to serve, and Marshall held the position for less than a year.

To hear more from Juliette Kayyem, tune in to Boston Public Radio above.