When I was a kid, I gave an opposing player some lip on the rugby field and he punched me in the face so hard and so quickly I didn’t even see it coming. One minute I was trash-talking him and the next minute I was on my butt, my head spinning, watching him run back to join the flow of the game. It was embarrassing. I remember how for weeks (months?) later I kept fantasizing about how I could have got back at him. I imagined clobbering him, humiliating him in front of others as he had done to me. The impulse to respond to violence with violence is primal. It’s almost involuntary. When we feel personally assailed we want to return fire, to make our attackers suffer as much, or more, than we have. It’s a very human, visceral reaction. Even when we see horrible acts of terrorism perpetrated in cities like Paris or London or Nairobi, that same impulse rears up. We feel threatened, and we clamor, “Do it back to them. Return violence for violence. If they’re trying to kill us we should kill them.” Whole nations can become inflamed by this hunger for revenge. After the September 11 terror attacks on New York and Washington, the US responded by bombing Afghanistan and invading Iraq.