She stood in just her bathrobe in the freezing basement of the Mariupol theater, coated in white plaster dust shaken loose by the explosion. Her husband tugged at her to leave and begged her to cover her eyes. But she couldn’t help it — Oksana Syomina looked. And to this day, she wishes she hadn’t. Bodies were strewn everywhere, including those of children. By the main exit, a little girl lay still on the floor. Syomina had to step on the dead to escape the building that had served as the Ukrainian city’s main bomb shelter for more than a...