1. "We sleep safely in our beds because rough men stand ready in the night to visit violence on those who would harm us." [ Attributed to Edmund Burke]
2. Be polite, be professional, but have a plan to kill everybody you meet.
One of the rules Maj. Gen. James Mattis gave his Marines to live by in Iraq, as quoted in Fiasco: The American Military Adventure in Iraq (2006) by Thomas E. Ricks; as excerpted in Armed Forces Journal