AP EXPLAINS: Why officers can't easily become Pentagon heads

WASHINGTON (AP) — Retired Marine Gen. James Mattis, President-elect Donald Trump's pick for defense secretary, will need more than the usual Senate confirmation before being able to take office.

U.S. law states that a nominee for defense secretary must have been out of the active-duty military for seven years.

Mattis, 66, retired in 2013 after serving as the commander of the U.S. Central Command.

The requirement was originally set by law in 1947, when Congress established a minimum of 10 years out of active duty. It was changed to seven years in 2008.

But there's a way around that restriction: Congress can approve a law bypassing it. That's what will have to be done for Mattis.

A look at why the limitation exists:

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