THERE I WAS, STANDING BAREFOOT in a field of fire with my socks and boots in my hands, obstinately refusing to run for cover until I had put my socks on. Jim was yelling something, but the machine guns kept drowning him out. Then came a brief lull, and I heard his voice loud and clear.

"Jon, fuck the socks! Run!"

It was the spring of 1983. Photographer Jim Nachtwey and I had teamed up to make one of the first trips inside Nicaragua with the CIA-backed contra guerrillas, who were fighting against the left-wing Sandinista regime. I was 26, and I'd never been under fire before. We had just spent an uneventful week with a contra platoon on an intelligence-gathering mission in the hills of northern Nicaragua. We moved around by night and, by day, hid and catnapped in thickets outside villages where the leader of our band, a tall, gangly, mustached man called "the Sparrow," rendezvoused with peasant collaborators.

Before we set out one evening, the Sparrow told us that at dawn we would reach a road where a Sandinista military convoy was expected to appear. He intended to ambush it. That night it rained torrentially, turning the ground to a mass of slick mud, and in the darkness I fell repeatedly. Before long I was completely covered in mud, and both my trouser legs had ripped all the way up to the crotch. They hung like a split skirt, and I felt miserable and ridiculous.

When we reached the road, the contras fanned out on a bluff, taking up ambush positions. The sky was just beginning to turn blue-gray. Everyone whispered and moved very softly.

I began changing out of my wet and ruined clothes. I took off my boots and socks and had just put on my spare trousers when a terrifying noise erupted. I looked up and, directly above my head, saw red tracer fire sweeping through the trees. It took me a moment to comprehend that we were being ambushed and that everyone around me had vanished. Getting ambushed is a shocking occurrence. When you're with people lying in wait, you have a sense of immunity to harm. But that was all turned around in a deadly second.

I finally spotted Jim and the others hiding in a shallow trench nearby, urgently motioning me to run and take cover with them. These instructions bewildered me; I still hadn't put on my socks, and I was determined to do so. So I yelled, "But my socks!" In that moment I learned a lesson that's served me well ever since: War, in all its manifestations, is essentially about fear—your own fear, collective fear, and how you handle that fear. Nobody knows until they've been under fire how they're going to react. In my case, the sock fixation was a form of shock.

Jim shouted something back, but I couldn't hear him over the gunfire. "What?" I said. He yelled back, but his voice was again drowned out. This exchange went on for what seemed like a long time, until I finally understood him telling me to run. I ran, barefoot, joining Jim and the others in the trench. When I got there, I realized that I'd brought my socks but left my boots behind. Jim retrieved them for me. And then we all ran like hell for the next five hours; we didn't stop until we reached the safety of the Honduran frontier.