A daring Ukrainian military push into Russia's Kursk region has seen Kyiv's forces seize scores of villages, take hundreds of prisoners and force the evacuation of tens of thousands of civilians in what has become the largest incursion into Russia since World War II.

In more than a week of fighting, Russian troops are still struggling to drive out the invaders.

Why did the Russian military seem to be caught so unprepared?

Russia's regions of Kursk, Bryansk and Belgorod share a 720-mile border with Ukraine. That includes a 152-mile section in the Kursk region. This frontier had only symbolic protection before Moscow invaded Ukraine in 2022. It's been reinforced since then with checkpoints on key roads and field fortifications in places, but building solid defenses has remained a daunting task.

The most capable Russian units are fighting in eastern Ukraine, where they have been pressing offensives in several sectors, with incremental but steady gains. Moscow has used the regions to launch airstrikes and missile attacks on Ukrainian territory but doesn't have enough land forces there.

Because of the porous border and manpower shortages, there have been earlier forays into the Belgorod and Bryansk by shadowy groups of pro-Kyiv commandos fighting alongside Ukrainian forces before they pulled back.

Russia's drones, surveillance equipment and intelligence assets are focused in eastern Ukraine, helping Kyiv to covertly pull its troops to the border under the cover of deep forests.

Retired Gen. Andrei Gurulev, a member of the lower house of Russia's parliament, criticized the military for failing to protect the border.

"Regrettably, the group of forces protecting the border didn't have its own intelligence assets," he said on a channel of his messaging app. "No one likes to see the truth in reports, everybody just wants to hear that all is good."

Ukrainian troops participating in the incursion reportedly were told their mission only a day before it began. That secrecy contrasted sharply with last year's counteroffensive, when Kyiv openly declared its main goal of cutting the land corridor to Crimea, which President Vladimir Putin illegally annexed in 2014. That military action failed as Ukrainian troops trudged through Russian minefields and were pummeled by artillery and drones.

Ukrainian troops faced no such obstacles entering the Kursk region.

Battle-hardened mechanized units easily overwhelmed lightly armed Russian border guards and small infantry units consisting of inexperienced conscripts. Hundreds were taken prisoner, Ukrainian officials said. The Ukrainians drove deep into the region in several directions, facing little resistance and sowing chaos and panic.

The operation resembled Ukraine's September 2022 counteroffensive in which its forces reclaimed control of the northeastern Kharkiv region after taking advantage of Russian manpower shortages and a lack of field fortifications.

Gen. Oleksandr Syrskyi, who led the Kharkiv operation two years ago, is now Ukraine's top military officer. Russian forces in Kursk answer to Gen. Alexander Lapin, who commanded Moscow's forces in Kharkiv in 2022 and was criticized for that debacle. But his ties to the chief of the General Staff, Gen. Valery Gerasimov, reportedly helped him survive and even get a promotion.

Syrskyi claims Ukrainian forces advanced across 390 square miles of the Kursk region, although it's not possible to independently verify what exactly Ukrainian forces effectively control.

"Thus far, the Russians have demonstrated tactical and operational shock, which has led to a slow tactical response and has allowed the Ukrainians to continue exploiting their breakthrough of the Russian defensive lines," said retired Australian Maj. Gen. Mick Ryan in an analysis.

The Russian military command initially relied on warplanes and helicopters to try to stop the onslaught. At least one Russian helicopter gunship was shot down and another was damaged.

At the same time, Moscow began pulling in reinforcements, which managed to slow Ukraine's advances but failed to completely block Ukrainian maneuvering through vast forests.

"Russia seems to do quite poorly when it has to respond dynamically in a situation like this," said military analyst Michael Kofman of the Carnegie Endowment in a podcast. "Russian forces do far better when they're operating with prepared defense, fixed lines, more on positional warfare."

Kofman noted the Russian reserves arriving in the Kursk area seemed to lack combat experience and had trouble coordinating with each other.

In one instance, a military convoy carelessly parked on the roadside near the fighting area shortly after the incursion began, and it was quickly hit by Ukrainian rockets.

"That's the kind of mistake the Russian forces along the line of control typically don't make," Kofman noted.

What are the risks for Ukraine of its incursion into Russia?

Kyiv remains tight-lipped about whether it intends to seek a foothold in the Kursk region or pull back into Ukrainian territory. The first option is risky because supply lines extending deep into the region would be vulnerable to Russian strikes, analysts say.

"The main risk is that the Ukrainians choose to try and consolidate and hold ground that lengthens the front line," said Matthew Savill, military sciences director at the Royal United Services institute in London.

Ryan, the retired Australian general, warned that "losing a large number of forces in this scenario also makes it a strategic and political liability."

That would "squander the very positive strategic messaging that has been generated by the Ukrainian surprise attack into Russia," he said. Ukrainian forces could try to retreat to a more defensible area near the border or fully pull back to Ukraine, he said.

The incursion already has boosted Ukraine's morale and proven its ability to seize initiative and take the war to Russian soil. Kyiv hasn't explained what the goal of the incursion is, but Ukraine's President Volodymyr Zelenskyy said he's considering setting up military offices in the Kursk region to help coordinate military and humanitarian efforts there.

"This Ukrainian operation 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," Ryan said.

What other developments are there in the war?

Ukraine ordered the evacuation of the Dontesk Oblast city of Pokrovsk on Friday, with the head of the city's military administration saying that Russian troops were "rapidly approaching" the city's outskirts and urging residents not to delay, according to CBS News partner network BBC News.

The Reuters news agency reported that three civilians were killed and five were injured in the Donetsk region in the last 24 hours.

Zelenskyy said on social media on Thursday that Ukraine's key frontline defensive considerations were "Toretsk, Pokrovsk, and others. These areas are currently facing the most intense Russian assaults and are receiving our utmost defensive attention," Zelenskyy said. "Priority supplies-everything that is needed-are being sent there."

contributed to this report.

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• War
• Ukraine
• Russia