The commander stepped over boxes stacked full of plastic drones and opened the lid on a new delivery. Inside lay the light gray fins of a mini plane, the latest addition to his arsenal of crewless aerial vehicles for fighting the Russian Army.

The 33-year-old leader of what an internal report declared Ukrainian Army’s best-performing drone unit, Senior Lt. Yuriy Fedorenko — popularly known by his call sign, Achilles — has been the main constraint on the Russian attempt to seize the strategic town of Chasiv Yar on Ukraine’s teetering eastern front.

For months, his drone teams, part of the 92nd Assault Brigade, have been filling a gap for other units of the army that have struggled with a shortage of troops and ammunition. The teams work day and night attacking Russian armor, dropping explosives on Russian positions and using their drones to ferry supplies to Ukrainian soldiers along the front line.

For the Ukrainians, holding Chasiv Yar is critical. Set on a ridge, five miles west of the destroyed city of Bakhmut, the town commands the heights above a crescent of industrial cities and villages that are home to roughly 200,000 residents.

Chasiv Yar is the gateway to the last part of the Donetsk region that is still in Ukrainian hands. If Russian forces were to capture the town, they would have the whole of the larger eastern area known as the Donbas within their grasp, long a goal of President Vladimir V. Putin’s. The cities of Kostyantinivka, Druzhkivka, Kramatorsk and Slovyansk, just miles beyond Chasiv Yar, have come under increasingly heavy bombardment in recent months.

“Without us, the Russians would be in Kyiv region by now,” Achilles said in an interview in a secret base set back from the front line. An exaggeration, perhaps, he said. (Kyiv, the capital, is far to the west.) Yet, he insisted, “Without the drones, we would lose.”