Seeing former president Bill Clinton in North Korea today on a mission to win the release of two journalists and perhaps raise other issues of concern in U.S.-North Korean relations immediately brings to mind a question:

Has any former president taken on such a job, presumably with the blessing of the current administration?

I called presidential historian Stephen Hess at the Brookings Institution to talk about the historical precedent. Hess basically said nothing quite like this has happened before.

We began with a conversation about the unique position Clinton is in as a former president and the husband of the secretary of State, and the big difference between his current mission -- which would seem to be in line with the Obama administration's wishes -- and the very independent type of diplomacy that former president Jimmy Carter has practiced over the years.

Then we discussed whether it's reasonable to believe that Clinton went to North Korea on a completely "private mission," as the White House has said. "Absolutely impossible," Hess says with a laugh...