When video footage of the Ukrainian incursion into Russia’s Kursk region began appearing on social media, a joke started doing the rounds with Vladimir Putin asking Stalin what he should do about the German tanks rolling towards Kursk. Stalin’s ghost responds that the recipe for victory is simple: send the best Ukrainian divisions into battle, like he did in 1943, and then ask the Americans for tanks and money. But neither of these options is available to Putin. He is now facing the Ukrainian army on his own soil, and regards the US as his primary enemy. Every year since the Russian invasion, Ukraine has surprised the world. First, at the very start of the war in 2022, its forces repelled a Russian assault on the capital, Kyiv. Then, in 2023, they liberated Kherson. Now, their tanks are rolling in to Russian territory. Ukrainian armed forces have been advancing for the past 10 days. They already control about 1,000 sq km of land and more than 80 settlements. Russian flags have been taken down; in the city of Sudzha, a military administration has been set up to govern the territory, and hundreds of prisoners of war have been captured. Kyiv’s territorial gains, in contrast to the size of Russia’s territory, are, of course, small. The Ukrainian army has taken the equivalent of about 10% of Greater London. The Kursk incursion may not be a decisive battle in this war, but it radically changes the prevailing narrative about how it is being fought and how it might end. It strikes at the root of the Kremlin’s strategy, which has been to have the west “freely reason to a conclusion that Russia’s prevailing in Ukraine is inevitable and that we must stay on the sidelines”, according to the Institute for the Study of War. The Kremlin has been successful with this narrative since the start of the war. As a result, the west only cautiously armed Ukrainian troops, and imposed restrictions on the use of its weapons systems, forcing Ukraine to fight with one hand tied behind its back. Meanwhile, Putin amassed troops and equipment and rained hell on the eastern frontline by launching glide bombs from Russian airbases, knowing Ukraine couldn’t strike back before the planes began their attacks. The Kursk offensive is flipping this narrative in Kyiv’s favour. It demonstrates that this war is not a stalemate. In recent months, the Ukrainian forces have carried out a number of dynamic operations. One of their most significant victories is clearing the Black Sea for commercial navigation by destroying 30% of the Russian fleet and pushing the remaining ships back to bases in the Sea of Azov. Meticulous drone strikes that destroyed sophisticated Russian air defences and targeted airfields on the Crimean peninsula have also been an integral part of this campaign. Deep inside Russia, more than 30 of the largest oil refineries have been under consistent attack since March. This has led to a drop in oil production and a 30% decrease in sales of diesel. Now, with Kursk, there are enemy forces occupying Russian territory for the first time since the second world war. Ukraine has apparently captured more Russian land than Russia has captured inside Ukraine since the start of 2024. Kursk also dispels the narrative that this war should only be fought on Ukrainian soil, and that, somehow, Ukraine could prevail by attacking Russian positions only inside its own borders. The US and Germany have been especially reluctant to allow the use of longer-range missiles to strike into Russia out of fear of escalation, so Ukraine has had to rely on its drones. This has played into Russia’s hands, allowing it to launch new incursions into Ukraine from its border regions. By stretching Ukrainian defences, Russia was hoping to make faster gains in Donbas. The Ukrainian army is now deploying similar tactics. The events of the past 10 days have demonstrated that Ukraine can walk across Russian red lines with impunity. Official US sources report that Russia has had to move some of its troops out of Donbas to stage a counterattack in Kursk. The offensive has also broken a taboo about the use of western kit inside Russia, which seems to be having an effect: the White House is reportedly considering providing long-range missiles to accompany the F-16 jets it has already sent. These could significantly complicate Russia’s attempts to reclaim its territory. Ultimately, Kursk provides the best possible response to the perhaps well-meaning but ultimately dangerous argument that Ukraine must end this war by exchanging its territory for peace, which was always premised on the belief that Ukraine couldn’t compete on the battlefield. Pushing Ukraine to compromise, to accept the annexation of its lands, and to stop seeking justice for this war, fundamentally misjudges the determination of the Ukrainian people and the strength of their army. This incursion shows that they are capable of military triumphs, and that western assistance pays off. It is the military theatre, not the negotiating table, that will define the outcome of this war. Will Ukraine be able to fully exploit its Kursk success? Its soldiers are consolidating the territory they’ve seized, digging defences and still advancing where they can. The Russians are preparing to launch a counterattack and build new defence lines to halt Kyiv’s advance. The episode has become a humiliation for Putin, and Kyiv will hope to capitalise on this, and to compel western governments to commit further resources. The new game begins. Orysia Lutsevych is deputy director of the Russia and Eurasia programme and head of the Ukraine forum at Chatham House