Under constant attack from drones attached to fibre optic cables, the soldiers scrambled in groups of two or three along hidden tracks or through fields, often walking miles on foot to get back into Ukrainian territory.

The Ukrainian retreat from the Kursk region, carried out in stages over the past two weeks, appears to mark the end of one of the most audacious and surprising operations of the conflict, and strips Ukraine of one of its few solid bargaining chips in possible peace negotiations with Russia.

For seven months, Ukraine held onto a chunk of Russian territory, including the town of Sudzha, which had a prewar population of about 5,000, the first time that a foreign army had occupied Russian land since the second world war.

Russia, with the help of North Korean troops, has been pushing Ukrainian forces back, and in recent weeks the pressure on Ukrainian positions has become overwhelming. On Wednesday, Vladimir Putin donned military fatigues to visit a command post in the region, and on Thursday, Russia announced it had regained full control of Sudzha.

While Ukrainian troops continue to hold a few remaining villages in Kursk, soldiers involved in the operation said it was probably only a matter of time before the retreat was concluded.

“The Russians are already pushing into Sumy region [in Ukraine], all the tasks now are defensive,” said Serhiy, a special operations commander who has recently left the region.

The demand by Putin that Russian troops create a “buffer zone” close to the border suggests a Russian offensive pushing back into Ukraine may be on the cards, and authorities have already evacuated several settlements close to the border.

The end of the seven-month operation has led to mixed assessments in Ukraine, with some saying it achieved many of its goals, and others wondering if it was a distraction from the main war effort and cost Ukrainian lives for no tangible gain.

“Kursk displaced the conflict on to Russian territory, and Russia used some of its best units to fight for it, but it also required a sizeable number of Ukraine’s elite units to hold the pocket,” said Michael Kofman, a senior fellow at the Carnegie Endowment for International Peace.

Now, the Russian offensive may change the dynamic of the war once again, at a moment when the US is pushing Moscow and Kyiv to sign a ceasefire agreement. “My worry is that this gives the Kremlin and the Russian army a new bout of enthusiasm and adrenaline,” said Kyiv-based political analyst Volodymyr Fesenko.

The initial incursion into Kursk came last August. It was planned by a very small circle to retain an element of surprise, and Kyiv did not even brief western allies until after the operation was under way. Troops arrived in nearby Sumy in small groups, and many stayed in rented apartments.

Documents recovered from Russian army positions showed that Moscow’s military planners had warned of the threat of an incursion for months, but when it came, Ukrainian troops were able to overrun Russian positions, taking hundreds of prisoners of war.

In Sudzha, officials and police fled without even destroying or taking away sensitive documents, leaving them for Ukrainian troops to seize and send to Kyiv for analysis.

Those with knowledge of the planning say Ukrainian officials were taken aback by just how well the initial stage of the operation went, as Ukraine took control of about 500 sq miles within a few days.

“At the beginning, the idea was just to create a diversion and to draw troops away from Donbas. But it went better than expected and suddenly we were digging in,” said one source.

“We have no wish to occupy this territory,” Zelenskyy’s aide Mykhailo Podolyak told the Guardian in late August. “Our tasks are to push Russian artillery and other systems further away, destroy the warehouses and other military infrastructure that is there, and also to affect public opinion in Russia,” he said.

But for the first time in the war, the shoe was on the other foot and Ukraine became an occupying power. The Ukrainian army took journalists on press tours of Sudzha, where stunned locals could not believe the war they had seen on their television screens had arrived on Russian territory. A historian from Sumy recorded a podcast in Sudzha’s small history museum, explaining how Sudzha and the rest of the region was historically Ukrainian land, a nod to Russian claims about Ukrainian territories.

At one point, there were plans to ask Russians who were fighting alongside Ukraine to enter the region and take over police functions. The idea would have been to troll Russia, which masked its 2014 intervention in Donbas as the work of Ukrainian separatists. However, the plans were quickly discarded as unworkable and incendiary.

The capture of hundreds of prisoners allowed Ukraine to offer an exchange and return home captured Ukrainians held in Russian jails, and the operation was also credited with providing a morale boost to Ukrainian troops and society after months of setbacks.

Finally, the longer Ukrainian troops held on to the land, the more it became clear that the endgame of the operation was now the chance to use Kursk as a card at future negotiations. Zelenskyy told the Guardian last month that Kyiv would “swap one territory for another”.

However, as time went on, Russia upped the pressure on Ukrainian positions in the region. By the beginning of this year, Ukrainian military planners could see Russia making a sustained effort to concentrate forces on the Kursk direction.

“Everything is going to Kursk,” said one military source in early February. “At the beginning they didn’t seem to pay that much attention to it, but then the North Koreans came, and then since the beginning of the year they are throwing everything at Kursk, perhaps to try to take it back before negotiating,” added the source.

Some of Russia’s best drone units were relocated to the Kursk theatre. Because Ukrainian forces can use radio jammers to block Russian FPV (first person view) drones, Russia has increasingly began using fibre optic drones, which unspool a thin cable over several miles and are immune to jammers.

“The fields we are retreating through are like a spiders’ web with all the fibre optic cables,” said Serhiy, the special operations commander.

He criticised the failure to build nets around the main road between Sumy and Sudzha to protect it from drones, as the Russians have done in part of the front in Ukraine’s eastern Donbas region. “Only in the last two weeks they started trying to do this, but now it’s too dangerous. They should have done it in September, October, when everything was calm,” he said.

Ukrainian officials say that while ultimately the operation will not provide the bargaining chip that had been hoped, it was able to change the narrative of the war, at a time when gloominess over Ukrainian prospects for any kind of advance had already set in. They say that while the withdrawal may be fraught, it has so far passed without enormous loss of life or stubbornly holding on to undefendable positions.

“After seven months, we simply withdrew. There was no encirclement,” said one senior security official, painting the operation as a success. “There were political goals, military goals, there was getting the Russians to move troops from Pokrovsk [in Donetsk] to Kursk. And we showed how shameful it is for the Russians, that they can’t fight Ukraine without North Koreans,” said the official.

Others had a more mixed assessment, noting that the offensive stretched Ukraine’s already understaffed forces, and did not slow the Russian advance in the east of the country. “Tactically it was successful, but the offensive changed little in the overall dynamic of this war, and failed to achieve its wider operational objectives,” said Kofman.