The rapid advance by Ukrainian troops into Russia's Kursk region represents the largest such cross-border attack by U.S. forces Kiev in the war that has been going on for nearly two and a half years. It exposed Russia's vulnerability and dealt a painful blow to the Kremlin.
The surprise incursion forced tens of thousands of civilians to flee the area as Russian armed forces tried to repel the attack.
For Ukraine, the cross-border raid provides a much-needed boost to public morale at a time when the country's manpower- and weapons-strapped armed forces are under constant Russian attacks along the more than 1,000-kilometer front line.
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A look at the Ukrainian invasion and its aftermath.
Kiev's troops entered Kursk Oblast from several directions on Tuesday morning and quickly seized several checkpoints and field fortifications along the region's 245-kilometer border with Ukraine, guarded by lightly armed border guards and infantry units.
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Unlike previous raids carried out by small groups of Russian volunteers fighting alongside Ukrainian forces, according to media reports, the Kursk Oblast incursion involved units from several battle-hardened Ukrainian army brigades.
Russian military bloggers reported that Ukrainian mobile groups, consisting of several armored vehicles each, are rapidly moving tens of kilometers into Russian territory, bypassing Russian fortifications and spreading panic in the region.
The Institute for the Study of War, a Washington-based think tank, said Ukrainian forces were able to penetrate the region to a depth of 35 kilometers. "Ukrainian forces appear to be able to use these small armored groups to carry out attacks from beyond the front line due to the low density of Russian troops in the border areas," said an analysis of the raid.
Ukrainian forces frequently use drones to strike Russian military vehicles and have electronic warfare assets to counter Russian drones and disrupt military communications.
While small Ukrainian mobile groups operated in the region without trying to consolidate their control, there were also reports that other troops had begun entrenching in the western part of the town of Suja, about 10 kilometers from the border, as well as in some other areas.
The Russian troops were taken by surprise and were unable to react quickly to the invasion. With most of the Russian army engaged in the offensive in eastern Ukraine's Donetsk region, few troops were left to defend the bordering Kursk region. The Russian units along the border consisted mostly of poorly trained conscripts who were easily overcome by the experienced Ukrainian units. Some conscripts were captured.
The manpower shortage initially forced the Russian military command to rely on warplanes and helicopters to try to stop the Ukrainian attack. According to Russian military bloggers, at least one Russian helicopter was shot down and another was damaged by advancing Ukrainian forces.
Later, Russian reinforcements, including elite special forces and battle-hardened veterans of the "Wagner" mercenary group, began arriving in Kursk Oblast, but so far they have been unable to push Ukrainian forces out of Suja and other areas near the border .
Some of the newly arrived soldiers do not have enough combat skills and suffered losses. In one case, for example, a convoy of military trucks stopped on the road near the war zone without sufficient precautions and came under Ukrainian fire.
The Russian Defense Ministry announced on Friday that Ukraine had lost 945 soldiers in four days of fighting. This claim cannot be independently verified. The agency did not provide data on Russian victims.
Officials in Kiev refrained from commenting on the cross-border raid, but President Volodymyr Zelensky praised the armed forces for their "actions to push the war into the territory of the aggressor.
"Ukraine proves that it really knows how to restore justice and guarantees exactly the kind of pressure that is needed - pressure on the aggressor," he said in one of his evening video addresses.
Zelensky's adviser Mykhailo Podoliak said on Thursday that the cross-border attacks would force Russia to "start to realize that the war is slowly advancing on Russian territory". He also suggested that such an operation could improve Kiev's position in possible future negotiations with Moscow.
"When will it be possible to hold a negotiation process so that we can pressure them or get something from them? Only when the war does not develop according to their scenarios", he declared.
Russian President Vladimir Putin called the incursion a "large-scale provocation", involving the "indiscriminate shelling of civilian buildings, residential homes and ambulances.
Russian authorities said at least five civilians, including two emergency workers, were killed in the Ukrainian attack in the Kursk region and nearly 70 people were injured. More than 76,000 people have been evacuated from areas affected by the fighting, officials said.
The deputy head of the Putin-chaired Security Council said the Ukrainian incursion highlighted the need for Moscow to expand its military objectives and seize more Ukrainian territory, including the capital Kiev, the Black Sea port of Odessa and other major cities.
Russia declared a federal state of emergency in Kursk Oblast and announced a counter-terrorist operation was underway in the area, as well as neighboring Belgorod and Bryansk Oblasts, giving local authorities more powers to quickly coordinate emergency response and clamp down of security.
Russian state propaganda has focused on the Kremlin's efforts to provide aid to displaced residents and played down the armed forces' lack of preparation for the attack.
By launching the raid, Kiev may aim to force the Kremlin to shift resources from the Donetsk region to the east of the country, where Russian forces have launched a multi-sector offensive and made slow but steady gains, relying on their firepower advantage.
At a time when Kiev's forces are trying to halt Russian advances in the east of the country, the swift cross-border raid demonstrates Ukraine's ability to take the initiative. He also hit out at the Kremlin, highlighting its failure to defend the country's territory and shattering Putin's narrative that Russia has been largely unaffected by the hostilities.
Despite initial successes, however, entering Russia could exhaust some of Ukraine's most combat-ready units and leave troops in the Donetsk region without vital reinforcements.
Attempts to establish a long-term presence in Kursk Oblast could prove a serious challenge for Ukrainian forces, whose supply lines will be vulnerable to Russian fire.
Military analysts believe that it is still unclear what Ukraine's operational goals are and how many military personnel have been deployed for the Kursk Oblast raid.
Michael Koffman, a military analyst at the Carnegie Foundation, wrote in his post on the X social platform that "a lot depends on what Ukraine has in reserve to include it in the operation, as well as how quickly the Russian Federation will organize itself to counter it".
Translation from English: Vladimir Arangelov, BTA