For the first time since World War II, Russian territory is under occupation.
Last week, Kyiv's troops poured into the Kursk region from several directions in a surprise attack.
Now seven days into the incursion, Russia still has not been able to push them back.
Ukraine had advanced about 30 kilometres into the region, and was now in control of 28 settlements, Russian authorities said.
It is Kyiv's largest cross-border assault since the start of the full-scale conflict, forcing more than 100,000 locals to flee.
The operation has been described as "bold", "ambitious", and "significant", and an embarrassment for Russian President Vladimir Putin.
It also shows that after almost 900 days of war, surprise is still possible.
But will it prove to have a big impact on the overall conflict?
How did Ukraine do it?
Last Tuesday's attack was shrouded in secrecy, catching even observers in the West — including the United States — by surprise.
Matthew Sussex from the ANU's Strategic and Defence Studies Centre, said Ukraine did well to keep the operation under wraps.
"It's just very good operational secrecy on the part of the Ukrainians, but obviously they were keeping forces in reserve to prepare for this," he told the ABC.
Ukraine acted quickly, reportedly sending several of its most "combat-ready" brigades to what was considered one of the weakest points on the border.
Mobile groups in armoured vehicles charged through, overwhelming checkpoints and field fortifications along the region's 245km frontier with Ukraine.
A lot of the success came down to the "very weak" borders, Professor Sussex said.
"And probably what it was intended to do was to bring home to the Russians that their borders aren't secure."
In Sumy, which borders Kursk, reporters in the region witnessed a steady stream of armoured personnel carriers and tanks moving towards Russia.
Strategist and retired Australian Army Major General Mick Ryan said attacking with a highly mobile force was essential to exploit enemy defences.
"This is different to the Russian dismounted attacks into Kharkiv in recent months," he said in an analysis of the attack.
He added that Ukraine's choice to allocate experienced units to the operation was "paying dividends with the depth of the Ukrainian penetration so far".
The operation has been supported by significant air defence, which is something Ukraine was unable to deploy to the same degree in its 2023 counter offensives.
Drones have been widely used to strike Russian military vehicles, and electronic warfare assets have been suppressing Moscow's drones and derailing military communications.
How has Russia responded?
There were suggestions Russian intelligence picked up on the plan, but the chain of command dismissed it as impossible, Professor Sussex said.
Now, the battle in the region has entered its seventh day, in what Mr Putin has called a large-scale "provocation".
Moscow's army rushed in reserve troops, tanks, aviation, artillery and drones in a bid to quash Ukraine's advance.
And Russian ally Belarus said it was bolstering its troop numbers at its border after Minsk claimed Ukraine had violated its airspace with drones.
But so far, Moscow has been unable to "throw them back," Professor Sussex said.
Although they have slowed, Ukrainian forces appeared to still be gaining ground.
Ukraine's top military commander said on Monday, local time, his forces now controlled 1,000 square kilometres of territory.
Acting Kursk Governor Alexei Smirnov reported to Mr Putin that Ukrainian forces had pushed 12km into the region across a 40km front and currently control 28 Russian settlements.
Earlier Russia's defence ministry said its forces had engaged Ukrainian troops near the villages of Tolpino and Obshchy Kolodez, about 25km and 30km from the Russia-Ukraine border.
In comparison, the Russian advance into Kharkiv this year penetrated about 8km into Ukraine.
Russia has been evacuating residents and imposed a sweeping security regime in three border regions.
More than 100,000 people have been evacuated, according to Russian authorities.
In the neighbouring Belgorod region to the south, the regional governor, Vyacheslav Gladkov, said on Monday that evacuations had also begun from the Krasnaya Yaruga District due to "enemy activity on the border" that was a "threat".
What are Ukraine's aims?
It was days before President Volodymyr Zelenskyy commented on the incursion.
In an address over the weekend, he said he had been briefed on Ukraine's actions to "to push the war out into the aggressor's territory", thanking troops who were proving the country knows how to "restore justice".
The exact aims of the operation remain unclear, and Ukrainian military officials appear to have adopted a policy of secrecy.
Professor Sussex said Kursk wasn't a particularly strategically important region, but it did have a large nuclear power plant.
It has historical significance for Russia and Ukraine, so taking territory in the region would be largely a symbolic gain.
"It's embarrassing for Putin. It's a wake up call to Russia, and it's a morale boost for the Ukrainians," Professor Sussex said.
Military analysts believe the main purpose of the incursion is to draw large numbers of Russian troops away from the frontline in the eastern Donetsk region.
Russian forces, which have a vast numerical supremacy and control 18 per cent of Ukrainian territory, have been advancing this year along the 1,000km front.
Mr Putin said Russia would carry on with its offensive in eastern Ukraine regardless of the Kursk incursion.
Mr Zelenskyy's adviser, Mykhailo Podolyak, said the cross-border attacks would cause Russia to "start to realise that the war is slowly creeping inside of Russian territory".
He also suggested such an operation would improve Kyiv's position in any future negotiations with Moscow.
Professor Sussex said Ukraine could try to capture a large number of Russian servicemen for prisoner swaps, but it was unlikely to be the main objective.
"To do that, they'd have to hold on for a very long time, and I'm not necessarily sure that they'll be able to do that," he said.
What does it mean for the conflict?
Major General Ryan described the operation as a "big swing" for Ukraine.
He said it "represents a very significant effort on the part of the Ukrainians to reset the status quo in the war, and change narratives about Ukraine prospects in this war".
The big question now is whether Ukraine can hold onto the Russian territory and for how long.
Russia was expected to "get their act together" and bring in more ground and air forces into the fight, Major General Ryan said.
Professor Sussex said Ukraine had taken a gamble moving into Kursk, but it had "to an extent" already managed to alter the dynamics of the conflict.
"It is really quite significant that Ukraine can take sizeable amounts of Russian territory," he said.
The next few weeks will be decisive.
Russia may be forced to divert a large number of resources to fighting the battle and securing its borders.
"It will be difficult for Ukraine to be in charge of large amounts of Russian territory for an enormous amount of time," Professor Sussex said.
"That said, if they have taken a significant amount of it, it will take the Russians a fair amount of effort to dislodge them."
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