Ukraine has seized 500 square kilometres of Russian territory in a lighting offensive into the Kursk region, creating panic among civilians.
There is significant concern in President Vladimir Putin's government after the daring attack in which several brigades of Ukraine troops have pushed more than 30km into Russia, seizing several towns and villages.
The occupation of Russian home territory could have a significant impact on Mr Putin’s authority, with Moscow scrambling to defend against the incursion that began on Tuesday.
“The use of highly capable Ukrainian combat forces in Kursk is either a brilliant countermove to shift the momentum in the war, or a strategic error which compounds the challenges in Ukraine’s eastern defensive operations,” wrote former US general Mick Ryan on X, formerly Twitter.
He added that the “strategic shock” will have an impact “on the status of Mr Putin at home and abroad" and raise the notion that he "can’t defend his own country”.
The Washington-based Institute for the Study of War said the substantial advances within Russia “would be a strategic blow” to Mr Putin’s “decades-long effort to cement a legacy of Russian stability, security and geopolitical resurgence”.
Russia was forced to move extra tanks, artillery and rocket systems to the Kursk region on Friday, as it battled for the fourth straight day to end the shock incursion.
Two days after military Chief of Staff Valery Gerasimov reported to Mr Putin that the advance had been halted, Russia's Defence Ministry said its forces "continue to repel an attempted invasion by the armed forces of Ukraine into the territory of the Russian Federation".
Interfax news agency quoted the ministry as saying Russia was sending in columns of reinforcements with Grad multiple-launch rocket systems, artillery and tanks.
Poor defences
In what appears to be a carefully planned assault, Ukrainian special forces conducted weeks of surveillance operations along the country’s northern border with Russia to ensure total surprise.
They detected a large section of poorly defended territory that lacked the belts of mines and ditches where the majority of Russian forces, about 520,000 soldiers, are fighting in eastern Ukraine.
If the operation’s momentum continues, then the city of Kursk itself, including its nuclear power plant 100km from the border, could come under threat.
More than 100 Russian soldiers have been taken prisoner and the Eastern Europe gas distribution terminal outside the town of Sudzha has been captured, with the Ukraine flag also flying from the town centre.
There were also reports of a chaotic Russian evacuation of the 18,000 citizens from the town of Lgov that lies 50km west of Kursk and 60km from the Ukraine border.
Fourth day of advance
Kyiv has yet to comment on the invasion but on Wednesday, Mr Putin convened a high-level meeting in Moscow at which defence chiefs falsely claimed the attack had been halted.
But the ISW reported that Russia’s Defence Ministry had removed some internet posts claiming the Ukrainian offensive had been stopped and soldiers were retreating “to avoid backlash from making demonstrably false claims about the scale and tactics of the ongoing Ukrainian penetration”.
Ukrainian forces have continued advancing into Russian territory for a fourth day, with elements of at least four of its most experienced brigades brought into the attack.
It remains unclear how much, if any, western-supplied equipment is being used in the assault, although Washington appeared to make clear on Friday that this was not an issue.
Multibrigade attack
The operation is also far bigger than the raids carried out mostly by pro-Ukrainian Russians on Belgorod region earlier this year and in 2023.
Some reports state that up to eight Ukraine brigades have been deployed, which would number between 30,000 and 40,000 personnel, with two high-quality formations confirmed, the 82nd Air Assault Brigade and the 22nd Mechanised Brigade. Given the current operational progress, more brigades could be drafted in to exploit the success.
It appears the Ukrainians are advancing on two axes, one to the north and one east, and have made the biggest advance by either side since late 2022.
There is some speculation that Ukraine’s forces could push behind the lines of Russian troops in the east, avoiding the mass minefields and taking them from the rear.
However, there is a danger that if the salient's flanks are not adequately protected, it could be vulnerable to a Russian counter-attack.
Drone attack
The Ukrainians appear to be using a new form of combined arms warfare, advancing with armoured vehicles under the protection of surveillance drones and electronic warfare jamming systems, then using huge number of FPV (first-person view) drones to attack positions.
In what is thought to be the first such attack, a FPV drone took down a Russian Mi-28 Havoc gunship helicopter by blowing off its rotor blade.
Another Ka-52 Crocodile attack helicopter was apparently destroyed along with a fighter jet.
Russia is now reported to be gathering forces for a counter-attack but it is unclear how many reserves it has, with so many troops committed to Ukraine where it is reportedly suffering 1,200 dead and wounded a day, with hundreds of armoured vehicles being lost, too.
Special forces are also pushing ahead of the advance, setting up ambushes of Russian reinforcements dashing to the new front, with video footage showing convoys of destroyed army trucks and dead soldiers.
Ukrainian forces have now reportedly destroyed at least two substantial Russian reinforcement convoys, including 19 vehicles in one.
Realities of war
There are several motivations behind the assault, with the opportunity to disrupt Russian attacks grinding through eastern Ukraine being foremost, said retired Brigadier Ben Barry of the IISS think tank.
“There is also this Ukrainian narrative that they quite clearly want to bring home the realities of war both to the Russian people and Russian elite,” he added.
If the Ukrainians hold the ground, their strategy could then be “to seize Russian territory as a bargaining chip for potential peace negotiations”.
From the air
The attack comes as Ukraine has received its first batch of about 20 F-16 fighters, which could be used to help defend airspace over the salient.
The UK’s Ministry of Defence highlighted in an X report how Ukraine’s continued drone and missile attacks on advanced Russian air defences and airfields has significantly depleted its capabilities.
It is the first time Ukraine has mounted a significant ground operation after its failed summer offensive of last year that came up against near impregnable Russian lines of mines, tank traps and trenches.
However, the Kursk border with Ukraine appears to have been only lightly defended, with conscripts and border guards and very few mines.
Total secrecy
That the Ukrainians managed to mount the attack in total secrecy is a severe intelligence failing on Russia’s behalf, especially at a time when the battlefield is now regarded as “transparent”, with so many surveillance drones airborne.
“This whole operation significantly damages the Kremlin's credibility,” said Brig Barry. “To achieve that level of surprise is highly impressive.”
The offensive could also prove a significant morale boost for the Ukrainian population, which has suffered months of Russian aerial bombardment and a gruelling defensive operation since January.
“Real buzz in Ukraine about Kursk,” wrote the respected blogger WarMonitor on X. “A moment of euphoria in a brutal war.”
