Behind the 1947 Law That Could Block Donald Trump’s Secretary of Defense Pick

Speaking at a rally in Cincinnati on Thursday night, President-elect Donald Trump said he will nominate retired Marine general James “Mad Dog” Mattis to become the next Secretary of Defense.

There’s just one catch, as TIME’s Mark Thompson has noted: One must be retired from the military for at least seven years to be “eligible” to be the Secretary of Defense, according to a section of the U.S. Code, and Mattis has been retired for three.

This requirement — which used to stipulate ten years until 2008— originates from the National Security Act of 1947, which President Harry S. Truman signed into law shortly after the end of World War II. That act create the role of the Secretary of Defense to coordinate the Army, Navy, and newly-created Air Force branches of the military. The law also created the Central Intelligence Agency (CIA).

The law was a product of its post-World War II time, according to Douglas T. Stuart, professor of Political Science and International Studies at Dickinson College.