Ukrainian forces have derailed Russia's plan to encircle and occupy the remaining Ukrainian-controlled territory of the Donetsk region this year, according to a new assessment by the Washington-based Institute for the Study of War (ISW). The think tank, relying primarily on open-source intelligence, concluded that a Ukrainian counter-offensive on the Lyman axis collapsed the northern arm of a Russian pincer movement designed to surround the so-called "fortress belt" — the defensive and logistical network protecting the Ukrainian-held portion of the Donbas.
The Russian campaign envisioned a deep envelopment, with one arm advancing from Lyman in the north and another from Dobropillia in the south-west, aiming to link up near Barvinkove in the Kharkiv region. Had it succeeded, Moscow would have moved significantly closer to its long-standing objective of occupying all of Donetsk, giving the Kremlin a tangible victory to present to the Russian public after years of costly warfare.
Operation Vivaldi: a tactical masterstroke
Ukraine's counter-offensive, dubbed Operation Vivaldi, systematically dismantled the northern pincer. Ukrainian forces advanced along the Ridkodub–Nove–Novomykhailivka line in the north and the Yarova–Drobysheve–Stavky line in the south, collapsing the salient Russia had built to threaten Sloviansk from the north-east. According to Andrii Biletskyi, commander of Ukraine's 3rd Army Corps, the first three phases of the operation, conducted in mid- to late September, liberated 176 square kilometres of territory. President Volodymyr Zelenskyy, speaking on 3 October, put the figure at between 125 and 140 square kilometres, while noting that Russian losses had risen sharply over the preceding two to three months.
The ISW suggested that the operation may have been even more successful than Kyiv's official figures reflect. The defeat of the northern pincer forced Russia to abandon its plans to advance on Sloviansk from the north-east and deprived it of positions intended to threaten the northern approaches to the fortress belt. Ukraine's retained control over the Lyman area also preserved key supply routes, and Russian units on other parts of the front may have been redeployed to reinforce the northern sector.
Russia's south-western advance from the Pokrovsk–Dobropillia area — the surviving arm of the original encirclement — now faces far more ambitious objectives on its own. The ISW regards the collapse of the northern pincer as an operational failure, as Russia has not just lost ground around Lyman but also a key component of its campaign plan for the fortress belt. Russian forces retain limited gains around Kostyantynivka and east of Sloviansk but have struggled to sustain the pace of advance in both areas.
The think tank argued that Russia's reliance on infantry infiltration, drones, and bombardment can produce gradual attrition but cannot generate the fast mechanised manoeuvres required to collapse Ukraine's defences at the scale Moscow needs. The situation remains fluid, the institute added, and Russian forces will probably adapt over time, but for now, Ukraine appears to have halted the advance. A further Russian mobilisation and technological adaptation could alter the balance, although neither would necessarily enable a breakthrough.
Putin's September figures challenged
The ISW also challenged claims, widely circulated in pro-government Russian media, that Russian forces occupied a further 1,301 square kilometres of Ukrainian territory in September alone — roughly two and a half times the figure reported in previous months. Based on geolocated photo and video footage, satellite imagery, and visually confirmed data, the institute calculated that Russian assault units had in reality been pushed out of at least 80 square kilometres in September. Russia's net retreat in September rose to 175.18 square kilometres, including areas it temporarily penetrated but did not hold.
President Vladimir Putin told the Valdai Club last week that Russia now occupies 89% of Donetsk region. The ISW puts the figure of Russian-occupied territory in the Ukrainian region at no more than 81%. The institute assessed that Putin's claims of Russian breakthroughs in September were "so far removed from reality that they resemble hallucinations."
The setback in the Donbas comes as Ukraine continues to face intense Russian strikes on civilian areas, with deadly attacks reported in Donetsk even as Ukrainian forces target Russian oil infrastructure. The broader conflict remains deeply intertwined with European security, as evidenced by the EU's ongoing, albeit contentious, energy ties with Moscow — the bloc spent €7.9bn on Russian Arctic LNG despite its phase-out pledge. Meanwhile, diplomatic efforts continue, with Miami talks aiming for a partial ceasefire as Ukraine braces for another winter of war.
For now, the ISW's analysis suggests that Ukraine has not only blunted Russia's immediate offensive but has also dealt a significant blow to Moscow's strategic ambitions in the Donbas. The coming months will test whether Ukraine can build on this momentum or whether Russia can regroup and adapt its tactics.
