The loss of the city of Avdiivka last month marked the end of a long, exhausting defense for the Ukrainian military. One brigade had defended the same block of buildings for months without a break. Another unit had been in the city for nearly two years. Ammunition was low, and the Russians conducted dozens of airstrikes every day. Kremlin soldiers came in waves. Then on Feb. 9, a battalion commander disappeared after getting into a car with two other Ukrainian soldiers. One was found dead. The officer and the other are missing. Within a week, Avdiivka fell to Russia.