Russia’s widely-anticipated winter offensive has begun. Aiming to extend its control over eastern Ukraine’s Donbas region, Russian troops are attacking north and south of Donetsk city. In the northern sector, around the city of Bakhmut, the Russians slowly are advancing—albeit at staggering cost. In the south, around Vuhledar, the Russians’ losses are just as steep—but they’ve made no clear gains that could justify the casualties. Vuhledar is turning into a meatgrinder for the Russian army, with enormous implications for the wider offensive. The latest Russian attack on Vuhledar—a town with a pre-war population of just 14,000 that lies a mile...