As the Russian army barreled toward Kyiv in the early days of Russia's wider war on Ukraine last February, the call went out for volunteers in the besieged capital city. These volunteers—thousands of them—formed a new territorial-defense unit. A year later, that unit—the 241st Territorial Defense Brigade—is in the thick of the fighting in what is, at present, the bloodiest sector of the wider war. The snowy hills and ruined city blocks in and around Bakhmut, 30 miles north of Donetsk, the seat of the separatist Donetsk People’s Republic in eastern Ukraine's Donbas region. The Russian army and its allies...