Vladimir Putin’s war plan was simple. Russian tanks would roll into Kyiv, while special forces seized key buildings and raised the Russian flag. The operation – to conquer Ukraine and to install a puppet government – would take around three days. The west would be horrified, of course. But – sooner or later – it would grudgingly recognise this new and great Russian reality. Two and half years after his full-scale invasion, however, the triumphant parade Putin envisaged on Kyiv’s Khreshchatyk boulevard has yet to happen. Victory has proved elusive. Ukrainians didn’t welcome “liberation” by their Slavic brothers in the way Putin’s spy agency predicted. They fought back. The country’s president, Volodymyr Zelenskiy, also failed to leg it and to follow Russia’s imperious script. After pulling out from around the capital in spring 2022, Russian troops have been going forward in the eastern Donetsk region. Putin, seemingly, had regained the initiative. His tactic: to bomb and steamroller towns and broken villages. The war, he calculated, would eventually end on Russia’s terms. Ukraine would be forced to give up the territory it had already lost – about 18% of the country – and more cities on top. The past 12 days have shattered the Kremlin’s strategic assumptions, borne from the fact that Russia has a bigger, more powerful army. On 6 August, Ukraine launched a surprise cross-border raid into Russia’s Kursk region. It was the biggest foreign incursion into Russia since the Second World War. Moscow had no inkling that this was coming. Ukrainians walloped a Russian border post and breezed into enemy territory. So far, the operation has been a stunning success for Kyiv, exceeding expectations. Mobile Ukrainian groups using US Stryker armoured vehicles and British Challenger 2 tanks have made rapid progress. They now control about 80 settlements, including the border town of Sudzha. The pace of their advance has slowed in the past few days. But Russia has been unable to halt this extraordinary mini-invasion. For Putin, these unforeseen events have been a personal humiliation. Videos sent from the war zone have been remarkable. Ukrainian servicemen filmed themselves ripping down Russian flags from municipal buildings. Hundreds of Russian conscripts – sent in as reinforcements – have surrendered. Wide-eyed Ukrainian journalists have toured Sudzha, reporting on a humanitarian aid delivery to civilians hiding in basements. When Russian troops overran Ukrainian cities in 2022, locals staged mass demonstrations. Near Kursk, by contrast, there has been apathy. Until now, most Russians have ignored the war, regarding it as something far away. Now, though, the conflict has reached their doorstep. Russian officials say 133,000 people have fled. Many feel abandoned. Slowly but surely, support for Putin’s “special military operation” is slipping. Zelenskiy and his generals appear to have overlapping goals. One is military: to force Moscow to divert troops from the east and to relieve pressure on the Ukrainian town of Pokrovsk and other vulnerable parts of the existing 977km-long frontline. This hasn’t happened. Thus far, Russia has redoubled its efforts in the Donetsk region and has scrambled conscript-heavy brigades from the rear, as well as from the occupied south of Ukraine. A further objective is to strengthen Kyiv’s bargaining position before possible negotiations. Putin has laid claim to four Ukrainian provinces, including the city of Zaporizhzhia and other chunks of territory that his forces do not control. These maximalist demands look ludicrous if he can’t defend Russia’s own flakey borders. Kyiv will want to keep its Kursk salient – until winter and beyond – with a view to a subsequent swap. Undoubtedly, the operation has boosted Ukrainian morale after a difficult, dark period and a bungled counteroffensive in 2023. It has cheered Ukraine’s international partners too. There are uncertainties ahead, with the US presidential elections in November and the possible return of Donald Trump. Kyiv’s message: with sufficient weapons we can win. Also, the west’s red lines – against the use of long-range weapons inside Russia – are meaningless when US-supplied Humvees trundle around the Kursk countryside. Whatever its eventual outcome, the attack has dented the myth of Putin’s invincibility. The president disappeared for much of last week. In crises, he typically keeps a low profile. Russian state media insist that all is well, and its forces are “repelling” terrorists and enemies. In reality, self-doubt is creeping in. Nobody in Moscow knows what Ukraine might do next – a second incursion in neighbouring Belgorod, maybe, or an audacious strike on Crimea? None of this means that Russia is likely to get a new head of state. No coup is looming. Now 71, Putin has been in power as prime minister or president for a quarter of a century, the longest stint by a Kremlin leader since Stalin. Rumours that he is ill – and possibly dying – turned out to be untrue. There is no obvious successor. Ever wary, he recently sent his former chief bodyguard Alexei Dyumin to oversee the defence of Kursk. Nevertheless, Putin’s grip is perhaps shakier than it seems. In summer 2023, the Russian warlord Yevgeny Prigozhin launched his own violent expedition. His Wagner mercenaries captured the Russian city of Rostov-on-Don and marched halfway to Moscow. Nobody stopped them. Prigozhin backed down and died two months later in a plane crash. The rebellion, though, revealed the brittle nature of Putin’s repressive regime. It is perfectly possible that Putin will soon deploy massive force and wipe out the intruders. As things stand, the war is going badly for him, his photo torn down from Kursk village halls and chucked in the bin. Karma, payback, hubris… the latest news does not conform to his messianic vision of a restored pan-Russia, encompassing Ukraine and Belarus. A keen student of history, Putin took part in celebrations last year to mark the USSR’s victory over Nazi Germany in the battle of Kursk. Now a second battle of Kursk is under way. It is bloody and fast-moving. Nobody knows how it will end. Some inside the Kremlin were against the invasion. One way to extricate Russia from this costly mess, they may conclude, is to remove the man in charge. Luke Harding’s Invasion: Russia’s Bloody War and Ukraine’s Fight for Survival, shortlisted for the Orwell prize, is published by Guardian Faber