Mattis has been sharply critical of President Barack Obama's policies on Iran and Obama's capping of troop numbers and campaign end-dates in theaters of war such as Afghanistan and Iraq. Mattis also appears to be a skeptic of the Obama-era policy of putting women into combat roles.

An intelligence officer and historian, Rayburn published a 2014 book, 'Iraq After America,' which is a must-read for anyone who wants to understand how Iraq descended into chaos in the years after the American troop withdrawal at the end of 2011.

The hard reality is that attacks by vehicles in public places are very hard to defend against in a free and open society. The best defense against such attacks is good intelligence, and that often comes from inside the Muslim community.

The dirty little secret of the intelligence world is that much of what you really need to know isn't exactly a secret anyway.