A swath of green runs through the city of Taiz, in Yemen’s southwestern highlands. From one of the craggy hills surrounding it, the strip resembles an urban planner’s modest attempt at a park. Close-up you can see that the beige of concrete blocks is split by tangles of trees and bushes sprouting from shell-pocked streets: this is no-man’s-land, nature reclaiming a place that has become too deadly for human life.

“The destroyed building there—that’s the beginning of the front line,” Abdulmalek al-Rabadi, a United Nations official who has worked to get humanitarian aid into the city, told me when I last visited a few years ago. We were looking down from an apartment block whose sides had been blown out by rocket fire, at territory controlled by the rebel group known as the Houthis. We had been told not to point, lest our gestures attract a sniper’s bullets.…