It was the dead of night, and the Ukrainian infantryman was writhing in a tree line from serious injuries to his legs, shoulder and lung.

His unit had told him by radio that they could not send anyone to evacuate him. The road to their base in the nearby city of Kostiantynivka had become a kill zone. “There were too many drones flying around,” recalled the infantryman, Oleh Chausov, as he described the experience.

Instead, he was told, the brigade would try to get him out with a small, robot-like tracked vehicle remotely operated from miles away and less visible to Russian drones than an armored carrier.

When the vehicle arrived, Mr. Chausov dragged himself aboard, his wounded legs dangling. But within 20 minutes, the vehicle hit a mine and blew up, he said. Miraculously, Mr. Chausov survived, crawled out and took shelter in a nearby trench.

He was back to square one, still trapped on the battlefield.

The operation in May — detailed in separate accounts from Mr. Chausov and an officer from his unit, the 93rd Mechanized Brigade, and captured in drone footage shared with The New York Times — underscores the dire conditions Ukrainian troops face defending Kostiantynivka.

The city stands directly in the path of Russia’s summer offensive in the Donetsk region of eastern Ukraine, which has seen Moscow’s troops make some of their biggest monthly territorial gains since 2022. Russia now controls more than two-thirds of Donetsk. But to seize the rest of the region, it must take urban centers still under Ukraine’s control and vital to its army logistics.

That makes Kostiantynivka a prime target. The city is the southern gateway to a chain of cities that form Ukraine’s last major defensive belt in Donetsk. Should it fall, nearly all cities farther north would come within range of Russian drones. It would bring Moscow closer to its long-sought goal of seizing all of Donetsk.

Russian forces have carved out a 10-mile-deep pocket around the Ukrainian troops defending Kostiantynivka, partly surrounding them from the east, south and west. Practically every movement in that pocket is targeted by Russian drones around the clock, according to a half-dozen Ukrainian soldiers and officers fighting in the area. Troops are often stranded for weeks without rotation or the possibility of evacuating the wounded.

“It’s extremely difficult to deliver supplies, to rotate troops — to do anything, really,” said Makas, an officer with Ukraine’s 12th Azov Brigade, using his call sign. Several service members interviewed for this article asked to be identified only by their call sign or first name for security reasons, and according to military protocol.

Ukraine is now bracing for Russia’s final push on Kostiantynivka although the battle could still take months to play out. The question is whether Moscow will launch a frontal attack, as in Bakhmut in 2023, or try to close the pocket in a pincer movement to force the Ukrainians to withdraw, repeating the strategy it applied when it took Avdiivka last year.

Either way, Ukrainian soldiers say Russia’s expanded drone strike capacity gives it an edge it did not have during previous assaults.

“Before, they could hit targets within two or three kilometers,” or less than two miles, said the commander of the unit operating crewless vehicles in the 93rd Brigade, who asked to be identified by only his first name, Oleksandr, according to military protocol. “Now, they’re striking every 10 to 20 minutes at a consistent range of 15 kilometers from the front line. Everything within that 15-kilometer zone is being destroyed.”

When asked about the turning point in Russia’s new drone campaign, Oleksandr and other Ukrainian soldiers all pointed to the same name: Rubicon, an elite Russian drone unit.

The group made its mark this year in the Kursk region of western Russia, where it helped push back invading Ukrainian forces by cutting off their main supply road with relentless drone strikes. Rubicon then redeployed to the area around Kostiantynivka this spring and applied the same tactic of systematically targeting roads, vehicles and antennas.

“The game changed when they came here,” said Rebekah Maciorowski, an American volunteer who heads the medical unit of a battalion in Ukraine’s 53rd Mechanized Brigade.

Speaking from a small field hospital in Kramatorsk, a city about 15 miles north of Kostiantynivka, Ms. Maciorowski was preparing small packages of antibiotic tablets that would be airdropped by drones to troops stranded at the front, one of the only ways to treat the wounded now that evacuations have become nearly impossible.

She assembled more packages than needed, anticipating that some drones would be shot down or disabled by Russian electronic jamming.

“There were times where I would send a medical package and the drone would drop out of the sky, and I would get another package, and another package — until one finally made it,” she said with a sigh.

Russian strikes around Kostiantynivka are so constant that Ukrainian troops avoid leaving their underground shelters altogether. “We joke that going to the bathroom is practically a heroic act now,” said Mykola, a Ukrainian serviceman fighting southeast of the city, who asked to be identified by only his first name, according to military protocol.

The Donetsk regional governor urged civilians to evacuate last month, calling it “a matter of survival.” But Russia’s dominance in the skies has made it increasingly difficult to evacuate the several thousand civilians still living in Kostiantynivka, which had a prewar population of 70,000.

Yevhen Tkachov, a volunteer with the Ukrainian aid group Proliska, which helps evacuate civilians, said he used to drive his pickup within five miles of the front. Now, he rarely ventures beyond 10 miles and has switched to using a smaller, less conspicuous car to avoid drawing the attention of Russian drones, which he said had targeted him in the past.

The road Mr. Tkachov takes to reach Kostiantynivka bears the scars of Moscow’s strikes. Burned-out cars and shattered buildings lined the road. One stretch is draped with a large net intended to intercept incoming drones. At the city’s northern entrance, a banner hanging from an overpass reads: “Welcome to hell.”

As Mr. Tkachov returned from an evacuation on a recent Friday, Tetiana Chubina, 74, stepped out of his car — pale, frail and trembling with emotion.

She said she had spent the past year holed up in her apartment in Kostiantynivka as the fighting closed in, and discovered the destruction in her hometown only when she left that morning to be evacuated.

“Around the train station, all the houses, all the factories are destroyed,” Ms. Chubina said. “Everything is smashed. It’s horrible.”

Russian forces often carpet-bomb cities to weaken Ukrainian defenses before launching assaults. Makas, the officer from the 12th Azov Brigade, said he did not expect a direct attack on Kostiantynivka that could drag Russian troops into bloody street battles.

Instead, he says Russia will try to bypass the city, pushing north from the two edges of the pocket it has carved out before closing the pincers on Kostiantynivka.

A near encirclement could force Ukraine to rely even more heavily on tactics developed to supply troops under dire conditions, including drone airdrops and deploying robot-like vehicles like the one that rescued Mr. Chausov.

Oleksandr, the 93rd Brigade officer, said the vehicles had proved their worth. Moving slowly at about 12 miles per hour, they are harder for Russian drones to detect. Each can carry up to 400 pounds of food, water and ammunition — far more than a soldier can haul.

Perhaps the most promising use is casualty evacuation, a task units cannot perform without risking more lives. Ms. Maciorowski said she had lost count of how many times someone got hit trying to rescue a wounded soldier, “and whoever goes to evacuate them also gets hit.”

After a vehicle evacuating Mr. Chausov hit a mine, his unit sent a second vehicle. It carried him under cover of darkness for several hours, finally reaching Kostiantynivka at dawn, passing a building still ablaze from a recent strike. There, a medical team pulled Mr. Chausov out and rushed him to a hospital.

Now recovering in western Ukraine, Mr. Chausov is still unsure how he made it out.

“There were so many drones,” he said in a recent phone interview. “It was a nightmare.”

Yurii Shyvala contributed reporting from Kostiantynivka, Ukraine, and surrounding towns.