PYSARIVKA, Ukraine—Six days after Ukrainian forces swept through the Russian border town of Sudzha in a lightning advance, a Ukrainian platoon carrying out a mopping-up operation stumbled upon a dozen Russian soldiers hiding in a butter factory.
The platoon leader, who goes by the call sign Yanyk, said his demand for the Russians to surrender was met by salvos of automatic-rifle fire. “So we eliminated them,” he said.
The speed and scale of this month’s Ukrainian advance—the first time a foreign military force has occupied Russian soil since World War II—left pockets of surprised and disoriented Russian soldiers trapped behind enemy lines.
Led by electronic-warfare units that jammed Russian communications and drones, units from Ukraine’s strategic reserve swarmed across the border on Aug. 6, seizing what Kyiv has described as 82 towns and villages in Russia’s Kursk region.
In all, Ukraine has taken 2,000 prisoners, according to a Ukrainian official, and seized about the same amount of territory that Russia, in grinding, high-casualty offensives, has taken from Ukraine since the start of the year.
Ukraine’s opportunistic assault has energized a nation battered by 2 1/2 years of war. It has also sent a message to Kyiv’s backers that the nation is still in the fight and can successfully mount complex and innovative offensive operations.
For Russian President Vladimir Putin, the Ukrainian thrust is an extraordinary reversal just as the weight of Russia’s massive war machine was beginning to wear down Ukraine’s defenses along the country’s eastern front.
Putin has vowed to drive Ukrainian troops out, but his forces have yet to mount a coherent counterattack and have had to pull units from Ukraine to respond. To dislodge the Ukrainians, Russia will likely have to send even more troops and pulverize its own towns and villages with artillery barrages, as it has done across a swath of Ukraine.
The challenge for Ukraine is how to capitalize on its early successes. Ukraine has committed thousands of troops. Sending in more could divert resources desperately needed to hold the Russians back on the main front line in eastern Ukraine. Russia is still advancing there against Ukrainian forces short on manpower and equipment.
To take full advantage, Ukraine would need a level of backing that the U.S. and some of its allies until now haven’t been willing to provide.
The Biden administration, which has said it didn’t know about the Ukrainian attack before it was launched, has barred Ukraine from using U.S.-provided longer-range missiles called ATACMS to strike Russian territory.
Washington isn’t sharing intelligence with Ukraine on targets inside Russia, said a senior U.S. official, who added that the Biden administration doesn’t want to be seen as enabling an attack into Russian territory.
The Ukrainians have used U.S.-supplied Bradley and Stryker armored vehicles inside Russia, taking advantage of the expanded latitude that the White House granted Ukraine to respond to Russia’s Kharkiv offensive in May.
This account of the Kursk incursion is drawn from interviews with dozens of Ukrainian soldiers involved, U.S. and allied officials and a person familiar with the operation, as well as videos verified by The Wall Street Journal and reports by Russian military bloggers.
Ukraine’s last attempt at a major advance, in summer 2023, ended badly. That counteroffensive gained only a few villages and cost the lives of thousands of experienced troops who couldn’t break through heavily fortified Russian defensive lines.
Since last fall, Russia has used its larger forces to grind out slow advances but at a heavy cost. Both sides are now dug into trenches along 600 miles of front lines surrounded by land mines and with artillery and drones ready to strike.
With the Kursk operation, Ukraine found a solution: go around.
Preparations began earlier this summer as Ukraine used drones to attack infrastructure in Kursk, from the power grid to ammunition and fuel-storage depots and surveillance equipment.
Members of an anti-Kremlin paramilitary group backed by Ukrainian military intelligence carried out raids across the border, probing for weak spots and conducting reconnaissance. Two weeks before the operation began, Ukrainian drones destroyed Russian observation systems at a border post on the road to Kursk.
For Mykola Toryanyk, a local Ukrainian official, the first rumblings came in early August. At night he could hear the sound of tracked vehicles on the rutted roads leading to the border near his district in Sumy province.
He assumed that Ukraine was pre-empting an attack by Russia, which had launched a fresh incursion into the nearby Kharkiv region in May. Ukrainian President Volodymyr Zelensky had warned that Moscow was massing troops for another thrust across the border.
Instead, Ukrainian forces were quietly building up for an attack of their own. Soldiers and equipment pulled from brigades across the country dissolved into the forests and villages near the border. The plan was shrouded in such secrecy that many of the men who would take part were unaware of their mission until it was nearly under way.
“We didn’t know,” said a slight Ukrainian artilleryman who goes by the call sign Shoom. His crew of eight men from Ukraine’s 44th Artillery Brigade had arrived in the Sumy border region days earlier after being pulled from the front line in the south.
At around 3 a.m. on Aug. 6, they unleashed an intense barrage from six M777 howitzers, supplied by Ukraine’s Western backers, which were set up along a stretch of the border aiming into Russia.
As Ukrainian assault forces smashed across the border in the early hours that morning, several Russian units found that their drones and communications systems weren’t working.
Ukrainian electronic-warfare units went into Russian territory ahead of the main mechanized assault forces to jam Russian equipment to stop Russian forces from pinpointing Ukrainian positions or intercepting their communications.
That unusual, early deployment—more like that of a reconnaissance unit—created a protective bubble around advancing Ukrainian assault forces.
Leading the charge were the 80th and 82nd air assault brigades, among Ukraine’s most powerful units, which are equipped with U.S. Stryker and German Marder armored vehicles.
The Western equipment had proved of limited utility on the battlefield in eastern and southern Ukraine, where dense minefields and drones make maneuver warfare all but impossible.
But in Kursk, Ukrainian mechanized troops raced down Russian roads with little resistance. Within hours, they had advanced miles into Russian territory, driving past towns and villages rather than attacking them head-on.
At a command center on the Ukrainian side of the border, members of the Nightingale Battalion, a drone unit, received coordinates. They sent explosive drones up to 10 miles into Russia to strike enemy dugouts, equipment, fortifications and soldiers.
The assault troops were advancing so fast it was hard to keep up with them, said one drone operator. There were few Russian armored vehicles to target because they had retreated north, he said.
“It’s not just because it was sudden,” said Kholod, another member of the drone unit. “It was deeply and well planned.”
Hours after the initial barrage, the artillery crew moved their M777 across the border and took up a position about 4 miles inside Russian territory. Adrenaline was high, said Shoom.
While assault troops surged ahead, other brigades poured into the Kursk region to mop up behind them on tanks and infantry fighting vehicles. In one village, a soldier stood on another’s shoulders to tear the Russian tricolor down from the local administration building.
Lightly armed Russian soldiers were still scattered in small groups, mounting localized resistance. “There’s a lot of them hiding in the forests and woods,” said Perchik, a soldier from the 22nd Brigade with a red-tinged beard.
To flush Russian soldiers out of a fortified position near the border, the brigade pointed the barrel of a tank directly at it, forcing them to surrender. Twenty-nine soldiers emerged in single file with their hands behind their heads. An additional 53 soldiers surrendered near the entrance to the gas-compression plant the same day, according to open-source videos.
Perchik had only ever taken one or two Russian prisoners at a time on the eastern battlefield, where he was fighting before. He took a selfie, wearing wraparound sunglasses and a huge grin in front of a line of fresh-faced POWs lying facedown on the asphalt. “They’re very young boys,” he said.
The elation was soon interrupted.
As they loaded the prisoners onto a truck to be taken to Ukraine territory, two Russian Lancet drones targeted Ukrainian equipment nearby, wounding four of the prisoners and knocking Perchik to the ground.
Russia, caught off guard, scrambled to pull together a response. With most of its military committed to Ukraine, Moscow initially relied on warplanes and helicopters to try to slow the Ukrainian advance, as well as lightly armed National Guard units. The Ukrainians moved forward air-defense systems to hold back Russian aircraft and used an explosive drone to down an attack helicopter.
On the fourth day of the incursion, a convoy of Russian military trucks laden with soldiers near the town of Rylsk was struck by rockets fired from a U.S.-provided Himars rocket system.
Video footage showed more than a dozen burned-out vehicles, some containing corpses. A U.S. official said Friday that the Biden administration didn’t believe Ukraine had moved Himars launchers into Russia but was most likely using them to fire precision-guided rockets from border areas in Ukraine.
Ukrainian artillery quickly moved deeper into Russian territory. On the outskirts of Suzha, the crew from the 44th Brigade targeted Russian equipment and soldiers on the main highway from Kursk.
Food, water and cigarettes were scarce, according to members of the crew, indicating the challenges Ukraine faces in resupplying troops in enemy territory. In the supermarkets, fresh produce had spoiled because there was no power, and the crew members were reluctant to shoot livestock.
“It’s hard the first week because you don’t know the roads,” said a stocky soldier with a gravelly voice, who makes daily trips in and out of Kursk to get supplies for his unit. The route is constantly changing because Russia is striking roads used by Ukraine, he said.
Most in demand are cigarettes and energy drinks. “You don’t sleep,” he said. “You’re on edge.”
An even bigger problem is glide bombs dropped by Russian warplanes. “They’re destroying their own cities,” said Shoom, the artilleryman, acknowledging that it gave him satisfaction after seeing Russia wreak destruction on Ukraine.
Ukraine has sought to counter the threat of glide bombs by attacking the airfields from which Russian warplanes take off. Ukraine attacked the military airfield in the Lipetsk region neighboring Kursk with drones in the early days of the incursion, destroying hundreds of Russian glide bombs, according to a Ukrainian official.
The Ukrainians were fighting what military experts call the close battle and the deep battle at the same time.
Ukraine is trying to cut off more Russian towns and make it difficult for Russian reinforcements to reach the front line by striking rail hubs and bridges. A bridge in the Glushkovo district, west of Sudzha, was destroyed Friday in a Ukrainian missile strike. Russia, meanwhile, said it had taken out two Himars systems in the Sumy region.
Nine days into the incursion, Ukraine appeared to have lost dozens of armored vehicles, according to counts by analysts who verify videos posted online. The human losses are unclear, but a spotter in the 44th Brigade crew said they were much lower than in last year’s counteroffensive, which he took part in.
Western military estimates suggest Ukraine has committed as many as 6,000 soldiers to Kursk and has up to 4,000 additional troops in support roles in the Sumy region.
To avoid weakening the front line, a substantial number of the troops came from a reserve force Ukraine had been building with Western encouragement for operations later this year and in 2025, according to the person familiar with the operation.
The Ukrainians are aware of that risk and are balancing so far, the person said.
Russia has pulled several understrength brigades from Ukraine in response, totaling up to 5,000 troops as of the middle of this past week, the person said. One brigade was from the Donetsk region, the focus of Russia’s current offensives, but others were from less essential areas.
Still, Russia might have to pull more troops out of Ukraine if it wants to take the territory back, which would require a larger force than the Ukrainians and would probably need to number more than 20,000 properly trained personnel, the person said.
In Moscow, Putin has held televised meetings with top defense and security officials in which he has ordered the Ukrainians expelled from Russia.
Satellite imagery, meanwhile, shows that Russia is hastily digging antitank ditches and other fortifications more than 30 miles from the border.
Jane Lytvynenko contributed to this article.
Write to Isabel Coles at isabel.coles@wsj.com and Michael R. Gordon at michael.gordon@wsj.com
