A bold Ukrainian military offensive into Russia’s Kursk region has resulted in Kyiv's forces capturing numerous villages , taking hundreds of prisoners and forcing the evacuation of tens of thousands of civilians in what has become the most substantial attack on the country since World War II.

After more than a week of combat, Russian troops are still struggling to expel the invaders.

Here's why the Russian military appears to have been caught off guard:

Russia’s regions of Kursk, Bryansk and Belgorod share a 1,160-kilometer (720-mile) border with Ukraine. This includes a 245-kilometer (152-mile) section in the Kursk region. This frontier had only minimal protection before Moscow invaded Ukraine in 2022. It's been strengthened since then with checkpoints on key roads and field fortifications in some areas, but building robust defenses has remained a significant challenge.

The most capable Russian units are engaged in combat in eastern Ukraine, where they have been mounting offensives in several areas, resulting in incremental but steady progress. Moscow has utilized these regions to launch airstrikes and missile attacks on Ukrainian territory but lacks sufficient ground forces there.

Due to the porous border and personnel shortages, there have been previous incursions into Belgorod and Bryansk by clandestine groups of pro-Kyiv commandos fighting alongside Ukrainian forces before withdrawing.

Russia's drones, surveillance equipment, and intelligence resources are concentrated in eastern Ukraine, enabling Kyiv to secretly deploy its forces to the border under the cover of dense forests.

Retired General Andrei Gurulev, a member of the Russian lower house of parliament, criticized the military for its failure to secure the border.

“Unfortunately, the force responsible for safeguarding the border lacked its own intelligence capabilities,” he stated on his messaging app platform. “No one enjoys hearing the truth in reports; everyone desires to hear that everything is fine.”

Ukrainian soldiers involved in the incursion were reportedly informed of their mission only a day before it commenced. This secrecy stood in stark contrast to last year's counteroffensive, where Kyiv publicly declared its primary objective of severing the land bridge to Crimea, which President Vladimir Putin illegally annexed in 2014. That military operation fell short as Ukrainian forces struggled through Russian minefields and faced intense artillery and drone attacks.

Ukrainian forces encountered no such difficulties entering the Kursk region.

Well-equipped mechanized Ukrainian units easily overpowered lightly armed Russian border guards and small infantry units made up of inexperienced conscripts. Hundreds were captured, Ukrainian officials reported. The Ukrainians advanced deep into the region in multiple directions, encountering minimal resistance and causing widespread chaos and fear.

The operation resembled Ukraine's September 2022 counteroffensive, where its forces regained control of the northeastern Kharkiv region after capitalizing on Russian manpower deficiencies and the lack of field fortifications.

Gen. Oleksandr Syrskyi, who led the Kharkiv operation two years ago, is now Ukraine’s top military commander. Russian forces in Kursk report to Gen. Alexander Lapin, who commanded Moscow’s forces in Kharkiv in 2022 and was criticized for that defeat. But his connections to the chief of the General Staff, Gen. Valery Gerasimov , reportedly helped him avoid repercussions and even get a promotion.

Syrskyi asserts that Ukrainian forces advanced across 1,000 square kilometers (390 square miles) of the Kursk region, though it’s impossible to independently confirm the precise areas effectively controlled by Ukrainian forces.

“So far, the Russians have displayed tactical and operational surprise, leading to a slow tactical response and allowing the Ukrainians to continue exploiting their breakthrough of Russian defensive lines,” said retired Australian Maj. Gen. Mick Ryan in an analysis.

Initially, the Russian military command relied on warplanes and helicopters in an attempt to stop the offensive. At least one Russian helicopter gunship was shot down, and another sustained damage.

Simultaneously, Moscow initiated the deployment of reinforcements, which managed to partially curb Ukraine's advances but failed to completely halt Ukrainian maneuvering through extensive forests.

“Russia appears to perform poorly when required to respond dynamically in such a situation,” said military analyst Michael Kofman of the Carnegie Endowment in a podcast. “Russian forces operate much more effectively when they have a prepared defense, established lines, and focus on positional warfare.”

Kofman observed that the Russian reserves arriving in the Kursk region appeared to lack combat experience and struggled to coordinate their actions with one another.

In one instance, a military convoy, parked carelessly on the roadside near the combat zone shortly after the incursion began, was swiftly targeted and hit by Ukrainian rockets.

“This kind of mistake is not something that Russian forces along the front lines typically make,” Kofman noted.

Kyiv remains silent about whether it intends to establish a presence in the Kursk region or retreat into Ukrainian territory. The first option is risky because supply lines extending deep into the region would be vulnerable to Russian attacks, according to analysts.

“The biggest risk is that the Ukrainians will try to hold onto and secure a larger front line,” said Matthew Savill, military sciences director at the Royal United Services institute in London.

Ryan, the retired Australian general, cautioned that “concentrating a large number of forces in this situation also creates a strategic and political vulnerability.”

That would "waste the incredibly positive strategic message that has been created by the Ukrainian surprise attack into Russia,” he said. Ukrainian forces could try to retreat to a stronger position closer to the border or fully withdraw to Ukraine, he said.

The incursion has already boosted Ukrainian morale and demonstrated its capacity to seize the initiative and bring the war to Russian territory.

“This Ukrainian operation marks a significant attempt by Ukraine to alter the current state of the war and change how the world views their chances of success,” Ryan stated.