What was widely seen as a diversion has become the main act as Ukrainian forces on Monday pushed deeper into Russia and again forced the Kremlin to scramble to respond.

Ukraine’s top military commander claimed his troops had seized control of a stunning 386 square miles of territory in Russia’s Kursk region. A week earlier, a surprise sortie ranked as the most significant cross-border attack since Moscow invaded Kyiv in 2022.

While fighting continues along the front line, the territory inside Russia’s Kursk oblast remains under control, Gen. Oleksandr Syrskyi said in a briefing to President Volodymyr Zelenskyy that was carried on Ukrainian social media.

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The initial attack flipped the narrative of the 30-month-old war. Russia’s larger and better-armed forces had been on the offensive for much of the year in regions around Kharkiv, Ukraine’s second-largest city. Ukrainian forces were trying to hold the line and falling back in some places along the more than 600-mile front in eastern and southern Ukraine.

Military strategists initially interpreted the Kursk operation as a diversionary maneuver designed to provide a quick morale boost before Ukrainian commanders withdrew the troops. With each passing day and advance against Russian forces, however, the Kursk fighting assumes far greater importance in the overall war.

Video captured by Russian war bloggers, who have been scathingly critical of the response to the invasion, shows Kursk residents boarding buses and other vehicles to escape the fighting. More than 100,000 Russians have been evacuated in the past week, officials in Moscow said.

Security analysts said warnings were issued about a Ukrainian military buildup inside the Sumy oblast on the border in the days before the Aug. 6 attack. Still, the operation caught Russia off guard, a source of particular pique for Russian President Vladimir Putin. Russian commanders have rushed reinforcements to Kursk, which had not previously been a site of major combat and was guarded by largely untested Russian military draftees.

“Russia left the border weakly defended, protected mainly by conscript and border guard units. The Russian defense was prepared to stop small-scale raids but not a sizable force,” said John Hardie, the Russia Program deputy director at the Foundation for Defense of Democracies think tank. “Ukraine exploited this surprise with a highly mobile force that advanced rapidly into Russian territory, overrunning or bypassing defensive positions near the border.”

Mr. Zelenskyy, who for days refused to confirm the Kursk invasion, said Monday that the Russian territory now under Kyiv’s control had been used to launch more than 2,000 artillery attacks inside Ukraine.

“Our operations are purely a security issue for Ukraine, the liberation of the border from the Russian military,” he said on his Telegram social media page. “It is quite fair to destroy Russian terrorists where they are [and] where they strike: Russian airfields, Russian logistics. We can see how this can benefit peace.”

Russia has set the agenda along the border so far, forcing Ukraine to expend materiel and manpower in purely defensive operations. The ongoing operation in the Kursk oblast has allowed Kyiv to seize the battlefield initiative in one area of the front line, according to an analysis by the Institute for the Study of War think tank.

“Russian President Vladimir Putin and the Russian military command likely incorrectly assessed that Ukraine lacked the capability to contest the initiative, and Ukraine’s ability to achieve operational surprise and contest the theater-wide initiative is challenging the operational and strategic assumptions underpinning current Russian offensive efforts in Ukraine,” the report said.

Russia has invested considerable resources in building fortifications along the international border but has not allocated the manpower and materiel to defend them effectively, the think tank’s analysts said.

“Sparsely manned and equipped border fortifications proved insufficient at preventing Ukrainian gains at the outset of the Ukrainian incursion into Kursk Oblast,” the institute said.

Propaganda coup

Mr. Putin has been sharply addressing his commanders. He told the nation that Ukraine likely began the Kursk attack for propaganda purposes and that the attackers would soon be thrown back across the border.

“It’s obvious that the enemy will keep trying to destabilize the situation in the border zone to try to destabilize the domestic political situation in our country,” Mr. Putin said. Russian forces have begun to “drive the enemy out of our territories and, together with the border service, to ensure reliable cover of the state border.”

Still, analysts say Russia will likely be forced to move additional manpower and equipment commitments to the border to prevent further Ukrainian cross-border incursions.

Moscow has accused Ukraine of launching missile strikes against Kursk but said most of the rockets were destroyed by their air defense weapons. The Kremlin claims that at least one of the missiles hit an apartment block and wounded 13 civilians. The report could not be independently verified.

“We strongly condemn these barbaric terrorist acts aimed at destroying civilian infrastructure and killing and intimidating civilians,” Russian Foreign Ministry spokeswoman Maria Zakharova said Sunday. “We have no doubt that those who organized and perpetrated these crimes, including their foreign sponsors, will be held to account. A tough response from the Russian armed forces will not take long in coming.”

Ukrainian officials didn’t give the White House an early notice about the incursion, but the mission doesn’t conflict with the Biden administration’s demands that U.S.-provided weapons not be used for attacks deep into Russian territory, Defense Department officials said.

“So far, we assess that they’re within the policy boundaries that we’ve set. The policies haven’t changed as it relates to the use of U.S. weapons,” said Maj. Gen. Patrick Ryder, Pentagon spokesman.

Mr. Putin’s grip on power appears secure despite the embarrassment of the Kursk operation, but more and more voices in Moscow are speaking up about the military’s shortcomings in a grinding war with a smaller, outgunned opponent.

Retired Gen. Andrei Gurulev, a member of the lower house of the Russian parliament, criticized the military for failing to protect the border, The Associated Press reported.

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

Mr. Zelenskyy used his briefing about Ukraine’s unfolding advance to again press Washington and its European allies to ease restrictions on launching attacks deeper into Russian territory.

“We need appropriate permits from partners for the use of long-range weapons,” he said. “This is something that can significantly bring a just end to this war, as well as save thousands of Ukrainian lives from Russian terror. Russia needs to be forced to make peace.”

• This article is based in part on wire service reports.