Southwest of the city of Bakhmut, Ukrainian soldiers Andriy and Borisych live in a candle-lit bunker cut into the frozen earth. For several weeks they have been confronting hundreds of fighters belonging to the Russian private military contractor Wagner throwing themselves against Ukrainian defenses. Disguised in a balaclava, Andriy recounts one seemingly endless firefight when they came under attack by a flood of Wagner fighters. “We were fighting for about 10 hours in a row. And it wasn’t like just waves, it was uninterrupted. So it was just like they didn’t stop coming.” Their AK-47 rifles became so hot from...