2. Be polite, be professional, but have a plan to kill everybody you meet.
Mattis drilled this rule into his Marines when they arrived in Iraq in 2003.
In this photo, outgoing commander of NATO troops in Afghanistan, US General David Petraeus, (2L) Chairman of the Joint Chiefs of Staff Adm. Michael Mullen (3L) the Commander NATO Allied Joint Force Command (JFC) Brunssum, General Wolf Langheld (C) US commander of the Middle East and Afghanistan, General James Mattis, (3R) and incoming General John Allen, (2R) salute during a change of command ceremony at ISAF headquarters in Kabul on July 18, 2011. (Getty)