On a recent morning deep in Ukrainian-occupied Russia, three soldiers from a Ukrainian special operations team jumped into their car, the back windscreen missing after being smashed out the previous day by explosives dropped from a Russian drone, and sped away in the direction of Ukraine. Six hours later, they would be in Kyiv, together with a precious cargo of documents stashed in boxes piled on the back seat, the fruits of a four-day mission into enemy territory for the trio. The documents included Russian interior ministry papers and military orders, seized from official buildings in Sudzha, the town at the heart of Ukraine’s surprise Kursk operation, and from abandoned Russian trenches nearby. “At the time it was all a blur, it’s only later when you come out that you realise where you’ve just been and what you’ve been doing,” said Artem, one of the three, talking at a roadside stop just hours after leaving Russian territory. Ukraine’s incursion into Russia, now in its fourth week, was shocking in its audacity, and has laid down an unexpected challenge to the Kremlin. Suddenly, it is Russian flags that are being pulled down from administrative buildings, Russian civilians who are taking shelter as soldiers of a foreign army patrols their streets, and Russia which is scrambling to prove it has control of its own long-established borders. Even as Ukrainian troops come under sustained pressure on other parts of the frontline, the dash into Russia has provided a much-needed morale boost inside Ukraine, after months of relentlessly bleak news. “They’re in a desperate David versus Goliath battle and this appeals to their rebellious spirit,” said one western diplomat in Kyiv, adding that the Kursk operation had boosted the mood among the political elite immeasurably in recent weeks. Part of the initial excitement came from sheer surprise. The Ukrainian president, Volodymyr Zelenskiy, and his top military commander discussed the plans in private and just a few people were brought into the circle. “Based on the experience of this war so far, the fewer people know about an operation, the more successful it will be,” said Mykhailo Podolyak, a key Zelenskiy aide, in an interview in Kyiv. “An extremely limited number of people knew ahead of time,” he added. In Sumy, the closest Ukrainian city to the border, locals noticed the city and surrounding areas filling up with military personnel in the weeks before the incursion, but did not know why. “There aren’t many rental options in Sumy, and people were asking around looking for anywhere free where soldiers could stay. It was only later, when the operation started, that the puzzle came together,” said Dmytro Tishchenko, the CEO of cukr.city, an online portal devoted to news and culture in Sumy. The troops themselves were also given no warning of what was to come. “We thought we were being transferred here to carry out defensive work against a possible Russian incursion,” said one soldier who has been in the area since a week before the assault began. Ukraine claimed last week to control nearly 1,300 sq km (500 sq miles) of Russian territory, comprising 100 settlements – mostly small villages, but including the town of Sudzha, home to 5,000 people before the assault. The road from Sumy to the border remains thick with military vehicles, with soldiers riding everything from motorcycles to tanks into Russia. In Sudzha, say Ukrainian soldiers, the streets are mostly deserted and a putrid smell hangs in the air, the result of produce rotting in the late summer sunshine. Many people fled further into Russia at the start of the offensive, but those who remain are now cut off, with no way out, no power and no mobile reception. The Ukrainian soldiers patrolling the streets are their only source of information. “We tell them Ukrainian forces have taken the city of Kursk and are marching on Moscow, and it’s time to learn Ukrainian,” laughed one soldier who had recently been in the city. Soldiers rotate out of Sudzha with trophies – ranging from Russian flags and posters seized from official buildings to T-shirts featuring Vladimir Putin taken from stalls at Sudzha market – but say they are not inflicting the same terror that Russian occupiers wrought on Ukrainian towns. Sudzha has now been under total Ukrainian control for a fortnight, but Russian drones remain a constant threat, and operating at the margins of the offensive can be a scary business. On one occasion, when clearing out a deserted Russian trench, Artem caught sight of what he thought was a fleeing Russian soldier. He was about to shoot when he realised someone had left a mirror in the trench, and he was seeing his own reflection. “You’re crawling through forests in the dark, and you realise that you’re absolutely alone, right inside enemy territory,” said Serhii, another member of the team. For now, Ukraine’s forward advance appears to have stalled, but Russia has not succeeded in retaking territory either. Kyiv says it has no interest in trying to annex Russian land, but for now wants to retain control of what it has taken. “We are not Russia, we don’t want to rewrite our constitution to add these territories … 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,” said Podolyak. Many in Kyiv also see the incursion as a message to Ukraine’s international partners, at a time when voices suggesting that some form of negotiations may be necessary in the medium-term future are growing louder. “Ukraine is trying to create leverage on Russia to have real negotiations, not a capitulation packaged as a negotiation,” said Alyona Getmanchuk, the founder of the New Europe Center in Kyiv. Podolyak first denied Kyiv had one eye on future negotiations, but then added: “Russia is not a rational country. It could be forced to negotiations, but for that you need [things like] the Kursk operation.” Getmanchuk said the operation, details of which Kyiv did not share with western partners beforehand, was also born out of frustration at repeated warnings about the risks of escalation by Washington. “The Kursk operation was a signal to forget about the supposed ‘red lines’ that Ukraine should not cross,” she said. Another goal of the operation was to capture Russian soldiers and use them as leverage to help free some of the thousands of Ukrainians held in Russian prisons. Ukraine claims to have captured nearly 600 Russian soldiers from inside Kursk region, many of them conscripts. A week ago, Ukraine swapped 115 of them for the same number of Ukrainians held in Russia. At a holding facility in Sumy region, Russian prisoners spoke of their shock at the war arriving to Russian territory and their unpreparedness for combat. Some had arrived in Kursk region only days or weeks before being captured. The Guardian spoke with more than a dozen Russian prisoners, who gave their consent to talk and were not monitored by prison guards while speaking, but is not quoting them directly due to international conventions around prisoners of war. “As a citizen of Ukraine I despise them, but I treat them how I want our prisoners to be treated in Russia,” said Volodymyr, the deputy head of the facility where they are held. “If we can use them to free our guys, then I am happy,” he added. While the Kursk operation has created a feelgood moment, there remains an acute awareness among many Ukrainian soldiers that things in the east of the country are looking increasingly bleak, as Russia steadily closes in on the city of Pokrovsk. If that advance continues, the whispered questions about whether the Kursk adventure was worth it are likely to get louder. For now, though, the operation remains a symbol of Ukrainian success on the battlefield. “We can use it to create a buffer zone alone the border to reduce attacks on us. And at minimum, we’ve got prisoners to exchange, and have given our population something to be happy about,” said Artem.