Five Russian soldiers, armed with rifles and grenades, crawled from their trenches into a low-lying ravine, ready to assault the Ukrainian position. At least one of them heard a hornet-like hum and looked up, into the lens of a drone looming above them, and realized that they had been detected.

Little happens in this war without the other side watching.

From an outcrop of jagged trees shedding their leaves before the onset of winter, a group of Ukrainian infantrymen swiftly retaliated with rifles and rocket-propelled grenades. At the same time, armed drones, operated miles away from the trenches, dropped grenades. The sound of incoming and outgoing fire merged, making a chorus of battle between trench lines just 150 meters, or nearly 500 feet, apart.

At first glance, the fighting resembled many of the thousands of battlegrounds dotting Ukraine, each with their own array of trenches and bunkers. But the Ukrainian soldiers said that a Russian breakthrough here, outside the city of Toretsk in eastern Ukraine, could have catastrophic consequences for their country’s defense against the Kremlin’s invasion.