A swift Ukrainian incursion into Russia’s Kursk region was the biggest cross-border raid by Kiev’s forces in the nearly 2½-year war, exposing Russia’s vulnerabilities and dealing a painful blow to the Kremlin.

The surprise attempt has prompted thousands of civilians to flee the region as the Russian military struggles to repel the attack.

For Ukraine, the cross-border raid offers a much-needed boost to public morale at a time when the country’s undermanned and underfired forces have faced relentless Russian attacks along the more than 620-mile front line.

A look at the Ukrainian raid and its consequences.

How did the Ukrainian attack unfold?

Kiev’s troops poured into the Kursk region from multiple directions early Tuesday, quickly overwhelming some checkpoints and field fortifications manned by lightly armed border guards and infantry units along the region’s 152-mile border with Ukraine.

Unlike previous raids carried out by small groups of Russian volunteers who fought alongside Ukrainian forces, the incursion in the Kursk region reportedly involved units from several battle-hardened Ukrainian army brigades.

Russian military bloggers reported that Ukrainian mobile groups consisting of several armored vehicles each quickly drove dozens of miles into Russian territory, bypassing Russian fortifications and sowing panic across the region.

The Institute for the Study of War, a Washington-based think tank, said Ukrainian forces have managed to push up to 20 miles deep into the region. “Ukrainian forces appear to be able to use these small armored groups to carry out attacks past the line of action due to the low density of Russian personnel in the border areas,” an analysis of the raid said.

Ukrainian forces have widely used drones to attack Russian military vehicles and deployed electronic warfare assets to suppress Russian drones and derail military communications.

While small Ukrainian mobile groups roamed the region without trying to consolidate control, other troops reportedly began digging in around the town of Sudzha about 6 miles from the border and in some other areas.

How has the Russian military reacted?

The Russian troops failed to react quickly to the incursion. With the bulk of the Russian army engaged in the offensive in Ukraine’s eastern Donetsk region, few troops remained to protect the Kursk border region. The Russian units along the border consisted mostly of poorly trained conscripts, who were easily overcome by elite Ukrainian units. Some conscripts were captured.

The lack of manpower led the Russian military command to initially rely on fighter jets and gunship helicopters to try to stop the Ukrainian attack. At least one Russian helicopter was shot down and another damaged by the advancing Ukrainian forces, according to Russian military bloggers.

Russian reinforcements, including elite special forces and hardened veterans of Wagner’s military contractor, later began arriving in the Kursk region, but they have so far failed to dislodge the Ukrainian forces from Sudzha and other areas near the border.

Some of the newly arrived troops lacked combat skills and suffered casualties. In one example, a convoy of military trucks carelessly stopped on the side of the road near the battle area and was hit by Ukrainian fire.

The Russian Ministry of Defense declared on Friday that Ukraine had lost 945 soldiers during four days of fighting. The claim could not be independently verified. The ministry did not offer data on Russian casualties.

What did the Ukrainian authorities say about the breach?

Ukrainian officials have declined to comment on the cross-border raid. In a video address to the nation late Thursday, Ukrainian President Volodymyr Zelenskyy avoided directly mentioning the fighting in the Kursk region. But he said that “Russia brought the war to our country, and it should feel what it has done.”

Zelenskyy adviser Mykhailo Podolyak said Thursday that cross-border attacks will make Russia “start to realize that the war is slowly creeping into Russian territory.” He also suggested that such an operation would improve Kiev’s hand in any future negotiations with Moscow.

“When will it be possible to conduct a negotiation process in a way that we can push them or get something from them? Only when the war is not going according to their scenarios,” he said.

What did the Kremlin say?

Russian President Vladimir Putin has described the incursion as a “large-scale provocation” that involved “indiscriminate shelling of civilian buildings, residential buildings and ambulances.”

Russian authorities said at least five civilians, including two ambulance workers, have been killed in the Ukrainian attack on the Kursk region and nearly 70 others have been injured.

Dmitry Medvedev, deputy head of the Putin-chaired Security Council, said the Ukrainian raid underscored the need for Moscow to expand its war aims to capture more of Ukrainian territory, including the capital Kiev, the Black Sea port of Odesa and other major cities.

Russia has declared a federal state of emergency in the Kursk region, giving local authorities more powers to quickly coordinate an emergency. Russian state propaganda focused on the Kremlin’s efforts to provide aid to displaced residents while downplaying the military’s unpreparedness for the attack.

What are Ukraine’s goals and how could the situation develop?

By launching the incursion, Kiev could aim to force the Kremlin to divert resources from the eastern Donetsk region, where Russian forces have been pressing multi-sector offensives and making slow but steady gains, relying on their superior firepower.

The swift cross-border raid, which comes at a time when Kiev’s forces are struggling to stem Russian advances in the east, shows Ukraine’s ability to take the initiative. It has also dealt a blow to the Kremlin, highlighting its failure to protect the country’s territory and shatter Putin’s narrative that Russia has remained largely unaffected by the hostilities.

But despite initial successes, the Russian incursion could cause attrition in some of Ukraine’s most capable units and leave troops in Donetsk without vital reinforcements.

Trying to establish a sustained presence in the Kursk region could be a challenge for the Ukrainian forces, whose supply lines would be vulnerable to Russian fire.

Military analysts say it remains unclear what Ukraine’s operational goals are and how many troops it has committed to the Kursk raid.

Michael Kofman, a military analyst at the Carnegie Endowment, said that “part of it depends on what Ukraine has available in reserve to throw into the operation and how quickly the Russian Federation organizes to counter.”