If in order to kill the enemy you have to kill an innocent, don't take the shot. Don't create more enemies than you take out by some immoral act.

I can't imagine that I would be asked that by the president-elect [Donald Trump], or then-president [Barack Obama]. But it's - I'm very clear. I voted for the change that put the Army Field Manual in place as a member of Congress. I understand that law very, very quickly and am also deeply aware that any changes to that will come through Congress and the president.

The most important six inches on the battlefield is between your ears.

What concerned me was that [James Mattis] played not only a critical role as a battlefield commander in Fallujah, but also, afterwards, when he was promoted to various other higher-ranking positions, he served as a convening authority in court-martial proceedings against various marines who had been accused of atrocities - for example, in the Haditha massacre, where a group of marines went on a killing spree after one in their unit was killed.

In this age, I don't care how tactically or operationally brilliant you are, if you cannot create harmony - even vicious harmony - on the battlefield based on trust across service lines, across coalition and national lines, and across civilian/military lines, you need to go home, because your leadership is obsolete. We have got to have officers who can create harmony across all those lines.

2017 is going to be another tough year for the valiant Afghan Security Forces and the international troops who have stood and will continue to stand shoulder to shoulder with Afghanistan against terrorism.

Be polite, be professional, but have a plan to kill everyone you meet.