Seven Januaries ago, I began walking across the planet. I wondered back then how human conflict would shape my route. As it turned out, I didn’t need to go anywhere to reach my first war. One engulfed my journey’s starting line in the arid Rift Valley of Ethiopia.

My local walking partner, Ahmed Elema Hessan, a balabat, or traditional leader, of the Afar pastoralist group, could see the front lines. I couldn’t.

“Go left!” Elema would say, as we scuffed across what appeared to be featureless desert. “No—more left!”

We were trudging north, making for the Gulf of Aden more than 200 miles away, all the while skirting invisible (to me) frontiers raided by “enemy” herders: the dreaded Issa. The Afar and the