Soldiers describe it as a dense jungle, suffocatingly hot and infested with mosquitoes, where you cannot see the enemy, or your own buddy, just three yards away.

It used to be one of Ukraine’s largest reservoirs, a wide body of water swelling the lower reaches of the Dnipro River in southern Ukraine.

Since the Russian Army blew up the Kakhovka Dam three years ago, draining the reservoir, the former flood plain has become a forest of reeds, scrub and willow thickets.

This inhospitable terrain is now a front line between the Russian and Ukrainian Armies, where drones cannot penetrate the cover of the tall weeds and soldiers hunt each other on foot in a deadly game of cat and mouse.

The Russian soldiers are trying to carve out trails toward their main target, the city of Zaporizhzhia, on the northern shoreline of the former reservoir; the Ukrainian soldiers lie in wait to ambush them.

“It is opaque, difficult, disorienting and very dangerous,” an operations commander with the call sign Divisiya said after overseeing an early-morning operation. Divisiya asked to be identified only by his call sign, in keeping with military protocol, as did most others in the battalion.

Four years ago, I joined the battalion, Bratstvo, to report on troops crossing the Dnipro River by boat to scout out and attack Russian positions on the southern bank. This summer, I was invited back to follow their operations in the overgrown reservoir basin from an underground command post as they fought a completely different kind of battle.

Now that the flood plains of the former reservoir have dried out and the summer foliage offers thick cover, Russian troops are trying to infiltrate through the basin, Oleksiy Serediuk, the battalion commander, said.

It is the latest chapter in a bitter, four-year contest for Zaporizhzhia, an industrial powerhouse with a prewar population of 750,000. Last summer, Russian troops advanced to within 20 miles of the city center. Ukrainian troops, spearheaded by military intelligence units like the Bratstvo battalion, succeeded in beating them back in recent months.

The Bratstvo battalion returned to the area last winter, tramping on foot through head-high reeds to scout the basin for ways to penetrate Russian lines.

In December, a Bratstvo reconnaissance squad encountered Russian units who had set up positions on a rocky outcrop, once an island jutting above the reservoir waters. In a clash, the Ukrainian forces killed four Russian soldiers and captured two others. Such forays allowed them to gather intelligence on Moscow’s plans, Mr. Serediuk said.

“They are trying to control the entire reservoir,” he said. “First, so they can operate there. Second, to reach the opposite bank.”

Soldiers who have fought in the area said the Russian military had deployed hundreds of troops into the reservoir, which is about 60 miles long and 15 miles wide.

Mr. Serediuk said, “They have already brought everything there, set up observation posts and staging areas for troop concentrations, which is exactly what they need to do.”

“We are looking for them, and they are looking for us,” he added.

Divisiya compared the Russian logistics operation to the Ho Chi Minh Trail during the Vietnam War, which became a vital lifeline under jungle cover for the Viet Cong to ferry weapons and supplies and personnel.

As the men of Bratstvo like to point out, the reservoir basin is a domain of infantry.

“In the reeds, it’s like the old kind of war, like at the beginning of the full-scale invasion, direct contact,” a squad commander with the call sign Apach said on his return from a two-week operation in the basin. “Once you step into a tree line, you’re gone.”

Drones have transformed the battlefield in many ways in Ukraine, allowing much of the fighting to be conducted remotely by pilots from bunkers. But they have proved of limited use over the dense vegetation of the reservoir, commanders and soldiers said.

“There were times when we spent three days looking for one of our own fighters, not to mention enemy soldiers,” Mr. Serediuk said. The battalion uses drones in a support role, to ferry supplies and sometimes to drop explosives in a general area to drive Russian soldiers into an ambush.

The difficulty for the Ukrainian battalion in tracking even its own soldiers was evident on a recent morning when a unit prepared to trek out from the reservoir to bring a Russian prisoner back to base. From a command post, an operations commander asked the unit’s troops to step out from the trees so that a drone pilot could track their progress.

Four figures emerged, one waved. Then they disappeared again into the trees. The only indication of their progress was the gentle movement of the tree tops.

Over several hours, the group came into view only when they crossed an area of sand and scrub or tramped through a patch of waist-high grass.

The Ukrainian unit twice ran into Russian soldiers and engaged them in firefights, Apach, the squad commander, said in an interview upon their return.

The first time, they came across a couple of Russian soldiers sitting smoking in some bushes. One of the Russians reached for his weapon and the lead Ukrainian opened fire and killed him, Apach said. The second Russian pulled the pin of a grenade until, after a few tense minutes, the Ukrainians talked him into replacing the pin.

They took him prisoner, and for three days he hiked through the forest with them back to their base. They ran into two more Russian soldiers who appeared as no more than shadows in the trees. Footage from Apach’s body camera showed him opening fire and then ducking down as bullets zinged through saplings.

He crawled backward, pulling the prisoner with him. “We started retreating because the contact was too close,” Apach said. “It was very dangerous.” They later found one dead Russian soldier, but the second shooter had escaped.

Two days later, the squad, still fired up with tension from the operation, arrived back at base with the prisoner.

A lot of the Russian troops seemed focused on evading attention, Apach said.

“Their task is to sit, listen and pass on information,” he said. “They don’t seem motivated to shoot or fight.”

Countless Russians have been killed in repeated assaults to regain control of a village, Prymorske, and other settlements south of Zaporizhzhia, and their bodies remain on the ground, said a commander with the call sign Stoic who oversees drone operations for the Bratstvo battalion.

Ukraine largely succeeded in stemming Russian assaults through the spring and summer, but Kyiv is already bracing for new offensives amid reports that Russia is preparing to enlist up to hundreds of thousands of recruits in a fresh mobilization.

Mr. Serediuk and his head of planning, an officer with the call sign Chornyi, are both veterans of multiple battles around Ukraine. Mr. Serediuk said that they had encountered some very skilled fighters among the Russian forces in recent months.

But Chornyi said that the quality of soldiers in the Russian Army had declined and that most troops deployed now were inexperienced, recent recruits.

Since the spring, Russian soldiers have been surrendering in larger numbers than in previous months, Chornyi added. He said that from interrogations of prisoners he had learned that they were less battle-hardened, but also that word had been trickling back through the Russian ranks that captives were not being tortured or killed, as Moscow had led soldiers to believe, but were being treated well in Ukraine.

“Many people are returning from captivity, and they talk about what captivity is really like and what the actual situation is,” he said.

In the Kakhovka basin, another motivation for surrender appeared to be the fear of being hunted in the reeds. Some of the Russian soldiers deployed into the former reservoir area were showing signs of panic as comrades were disappearing, Chornyi said.

Mr. Serediuk visited the Russian prisoners the battalion holds to send a message that he said he hoped they would take back with them on their eventual release.

He told them that Bratstvo was an Orthodox Christian battalion that respected prisoners and that he did not enjoy seeing so many Russians dying on the battlefield.

“So many men are sent into battle, and they die, and die, and die,” he told one of the prisoners. “My question is: For what?”

Kamila Hrabchuk and Oleksandr Chubko contributed reporting.