Ukraine's Operation Vivaldi has pushed Russian troops back from the Donbas Fortress Belt towards the Zherebets and Siverskyi Donets rivers, consolidating gains made earlier this month in a stunning stategic reversal that now makes Russia's chances of capturing the rest of Donbas by winter look remote.

The Third Army Corps of Brigadier General Andrii Biletskyi announced the counteroffensive north of Lyman on September 15 and says it has since recovered about 125 sq km, retaking Shandryholove and Derylove and taking full control of Drobysheve, Kyiv Post reported on September 23. "As promised, I am reporting good news," Biletskyi said on September 20 as he announced the second phase of gains, Kyiv Independent reported.

Russia had been closing in on the belt, the chain of fortified cities of Sloviansk, Kramatorsk, Druzhkivka and Kostiantynivka that has anchored Ukraine's defence of Donetsk region for years, and in July looked to be in a good position to take it over the winter. Losing that line could have decided the war. Vivaldi has disrupted that advance: the Institute for the Study of War (ISW) said on September 16 that Ukraine's gains north of Lyman had made Russia's plan to swing around the belt from the north "unfeasible", Euromaidan Press reported. For a Kremlin that needs a win soon, with its budget deficit and a stalling economy, that is a serious setback.

False Russian Ministry of Defense claims of encirclement of Dobropillia as of September 24, 2026. Source: Institute for the Study of War

Encircled on paper

Russia's Defence Ministry said on September 24 that units of its Centre Grouping were destroying encircled Ukrainian forces in the Dobropillia area while advancing towards Oleksandrivka, and published a map showing the city surrounded. The ISW's own map, in its assessment the same day, puts the nearest Russian forces 10.2 km from the city's southeastern outskirts, Euromaidan Press reported on September 25. The Ukrainian military says the city remains under its control.

The ISW said it had seen no evidence to support the encirclement claim, which it described as partly intended to divert attention from Ukraine's elimination of the Russian salient north of Lyman. It did observe small groups of Russian infiltrators operating east and northeast of Dobropillia on September 19 and 20, but did not assess that they had consolidated positions. One Russian milblogger suggested the "encirclement" might mean no more than fire control over Ukrainian supply roads west of the city.

The think tank blamed what Russian milbloggers themselves call "beautiful reports", a culture of inflated claims that it said has degraded the Defence Ministry's own picture of the battlefield. The ministry's statement described Dobropillia as a "strongly fortified settlement" whose capture would threaten Ukrainian supply lines running from Dnipropetrovsk region to the Fortress Belt. Biletskyi has said Russia's aim was to bypass the fortified cities through Dobropillia and link up with troops advancing from north of Lyman, rather than storm them head-on.

How close Russia came

Pokrovsk, the key logistics hub southwest of the belt, went first. Chief of the General Staff Valery Gerasimov told President Vladimir Putin on November 30 that it had been taken, and the city fell to Russia at the end of last year after months of street fighting.

By the summer Russian forces were advancing on Kostiantynivka, the southern anchor of the belt, with small groups infiltrating its outskirts and Putin saying the city was close to falling. "The city's fall seems to be more of a question of time," Emil Kastehelmi of Finland's Black Bird Group said in early July. Russian troops also took high ground between the Bakhmut-Siversk line and the belt that would bring Sloviansk and Kramatorsk into artillery range.

Kostiantynivka held. At the end of July, Ukrainian forces were still present across almost the whole city, according to open-source trackers, but the pincers were in place. A southern thrust through Dobropillia and a northern one from the Lyman salient would have closed a ring behind the four cities without Russia having to storm any of them.

Back to the river

Vivaldi cut off the northern pincer. Ukrainian assault groups are now moving down several routes from Lyman towards the Zherebets River, trying to secure its western bank and finish clearing the village of Zarichne, according to open-source mapper AMK Mapping. Troops coming from Ozerne are clearing the remaining Russian positions in Yampil and attacking the last Russian bridgehead over the Zherebets from the south, while others advance east along the railway line from Lyman.

Ukrainian assault routes from Lyman towards the Zherebets River, Zarichne, Yampil and Torske. Source: AMK Mapping

The gains have reopened the route for Ukraine to send reinforcements south across the Siverskyi Donets into the area east of Mykolaivka and Sloviansk, flanking Russian positions from the north, the mapper said. Its caveat is two large stretches of forest where Ukraine has very few positions, which remain open to Russian infiltration that could disrupt the movement of assault groups and reinforcements, especially if the Russian bridgehead survives.

Russian forces are carrying out emergency counterattacks south of the Siverskyi Donets, east of Mykolaivka, and have likely re-entered Starodubivka, according to Hudson War Mapping, which called the situation "extremely confusing and complex".

The Lyman-Sloviansk front around Yampil, Kryva Luka, Kalenyky and the Siverskyi Donets. Source: Hudson War Mapping

French open-source analyst Clement Molin puts the front of Ukrainian gains at nearly 90 km, from Borova to Sloviansk. "The Ukrainians are on the verge of securing Sloviansk - one of the two key cities in the Donbass - by pushing back the front line," he wrote on September 25, adding that Russia's Zapad grouping had suffered "a major defeat" and that much of the ground retaken has yet to be announced. Military analyst Delwin compared Vivaldi to Ukraine's Kursk incursion of 2024, launched at a similar point in the year, and said fierce infantry skirmishing continues across the Lyman-Sloviansk front, mostly in the grey zone.

None of the mappers' claims can be independently confirmed, and official figures are broader. Across the whole front, Russia has taken about 890 sq km this year and Ukraine has retaken about 800 sq km, President Volodymyr Zelenskiy told The Wall Street Journal in an interview published on September 22, against 3,000-4,000 sq km a year of Russian gains in earlier years.

In ones, twos and threes

Since December Russia has largely dropped the costly frontal assaults by massed infantry that drove up its casualty figures, and instead sends soldiers forward alone or in groups of two or three to slip through the Ukrainian line, take up positions between or behind the defenders and attack them from there. Small groups are also harder for Ukraine's drones to find and kill than a column of armour. In July Russian assault groups were already using summer vegetation to infiltrate Ukrainian positions before bringing up heavier weapons.

The tactic feeds on Ukraine's manpower crisis. With too few infantry to hold a continuous line, there are gaps of up to a kilometre between Ukrainian positions, covered by drones rather than soldiers. Infiltration has become a standard Russian tactic, a creeping "ink-blot" approach in which Ukrainian control is eroded by handfuls of Russians settling in behind the lines, researcher Chris Owen said on September 25. It can work because Ukrainian forces are thin on the ground, he said, but constant aerial surveillance and fast drone strikes have made it very costly.

Nikolai Gavrilov, a Russian military volunteer and milblogger, admitted that Lyman, which the Russian government had described as the scene of "house to house clearing" before Vivaldi, is firmly in Ukrainian hands and that infiltrators sent there are usually killed, captured or desert, in a post translated by Owen.

"The city was supposedly captured, but the reality is different. There is still an administration, police, and volunteer organisations of Ukraine present in the city," Gavrilov wrote. "Every day, units of the Ukrainian Armed Forces conduct patrols, using drones to monitor all the abandoned buildings, searching for our soldiers who may be trying to escape."

"Day after day, these soldiers, who have managed to establish contact, are killed. I know one fighter who managed to stay in contact for a week - the civilians betrayed them to the Ukrainian Armed Forces, and he was killed," he wrote. "Many fighters, after receiving orders to go to Lyman, simply turn off their radios and desert."

Desertion was already a growing problem for Moscow, with more than 28,000 Russian servicemen convicted of going absent without leave by May 2025, Meduza reported in July. Russian infiltrators are now also trying to get behind Ukrainian lines east of Lyman that they believe are poorly consolidated, Hudson War Mapping said.

A war Moscow needs to win soon

Losing the prospect of the belt this winter hurts a Kremlin running short of time and money. Russia's federal budget deficit reached RUB5.8 trillion ($68bn), or 2.5% of GDP, in the first eight months of the year, and Finance Minister Anton Siluanov has conceded that the full-year gap will overshoot the planned RUB3.786 trillion ($45bn) and could be at least double the original target. Industrial output grew just 0.1% in the first seven months, oil export revenues fell to $13.8bn in July from $15.8bn in June, and the Central Bank of Russia paused its rate cuts at 14% on September 11 with inflation still above 6%.

Zelenskiy estimated Russia's irrecoverable losses this year at about 270,000 troops, a figure that could not be independently verified. "He is neither winning nor losing, but his society is losing people and money for the sake of some 100 kilometers," he said.