On the day Ukraine launched its daring incursion into Russia’s Kursk region, the Federal Security Service, the Russian agency most responsible for protecting the border, played down the seriousness of the operation. Calling it “an armed provocation,” the agency said its forces were working to push the Ukrainians back.

That was nearly a month ago. Since then, Ukrainian forces have occupied a small but significant patch of Russian territory and killed or captured hundreds of Russian troops, according to officials, analysts and satellite imagery.

President Vladimir Putin has said an assessment of the failures in Kursk would be made only after the situation in Russia’s border region had stabilized, but intelligence experts say that a large measure of the responsibility rests with the Federal Security Service. Despite its networks of agents and vast budget, the agency, known as the FSB, first failed to anticipate the Ukrainian incursion and is now struggling along with the Russian army to dislodge a sizable Ukrainian fighting force.

There were clear signs that something was brewing. Days before the incursion, Russian bloggers, citing local residents on the Ukrainian side, reported a massive buildup of Ukrainian armor. But if it noticed anything amiss, the FSB failed to prepare sufficiently. When Ukrainian troops charged across the border Aug. 6 and pushed dozens of miles into Russian territory, they encountered almost no resistance.

“We are talking about many, many units which should have seen something and they failed,” said Andrei Soldatov, an author who has spent his career researching Russia’s security services.

The FSB is a muscular, authoritarian version of the American FBI, with a broad national security mandate that includes defending against threats from within Russia and in former Soviet republics.

But the agency is hindered by infighting, rivalries with other security agencies and an aversion to delivering bad news to Putin. Particularly since the full-scale invasion of Ukraine in 2022, the FSB has also been distracted by a large-scale crackdown on internal dissent.

As a result, the agency has suffered a series of damaging intelligence failures since the start of the war, Western officials and experts say. Kursk was just the most recent.

In March, the agency ignored specific warnings from the United States and failed to prevent a terrorist attack on a Moscow concert hall that killed more than 140 people.

Ten months earlier, the agency was caught off guard when Yevgeny Prigozhin, the leader of the Wagner mercenary group, launched a rebellion aimed at toppling Russia’s military leadership. And it was the FSB that famously informed Russia’s military that its troops would be greeted with flowers when they launched their invasion of Ukraine in February 2022.

In each instance, though, there were few visible repercussions from Putin, and there is no indication that there will be after the debacle in Kursk.

A week after Ukrainian troops crossed the border, the agency’s longtime director, Alexander Bortnikov, made a public appearance in which he seemed to place blame for the incursion on local municipalities in the border region, even as he made assurances that everything was under control.

“At the moment, all necessary measures are being taken to protect our citizens,” he said.

Putin on Sept. 5 sought to downplay the incursion. Speaking at an international conference in Vladivostok, Russia, he said that Ukraine had made a mistake in deploying “fairly large and well-trained units” to the Kursk offensive and that Moscow’s “first-priority goal” was its military’s offensive in eastern Ukraine.

But Ukraine’s continued presence has dealt an embarrassing blow to Putin, whose power in large part rests on the projection of strength and stability.

The FSB, as the primary domestic successor to the Soviet-era KGB, is perhaps the most important pillar of Putin’s power, responsible for suppressing dissent within the Russian population and keeping other government entities, including the military, in line. Despite the failures, Putin simply cannot do without the FSB, Soldatov said.

“It’s the lesson he learned from Stalin,” Soldatov said. “During war, you cannot punish your agencies because it might be more dangerous for you.”

Other agencies besides the FSB, including the military intelligence service and the national guard, as well as the army, have intelligence-gathering resources that could have detected the Ukrainian buildup. Part of the problem, experts say, is a lack of coordination within the military and intelligence operations that is likely to have impeded any Russian response.

It is a problem that has bedeviled the Russian war effort from the first days of the invasion, but one that Putin appears reticent to address.

This is in part by design, said Douglas London, a former CIA clandestine officer. Putin seems to be as wary of his own military and intelligence services as he is of the Ukrainians, London said, and has created a system of competition among them so that no single entity can become too powerful.

Instead of collaborating, part of the FSB’s job is to spy on other government agencies, particularly the military, which Putin, as a former KGB officer, was trained to distrust, he said.

“If agencies are collaborating, they could also collaborate against him,” London said.

The FSB was not the only intelligence service to be surprised by the Ukrainian incursion. Planning was carried out under such strict secrecy that even Ukraine’s allies did not know about it, officials in Ukraine and Washington said.

Senior members of Ukraine’s own intelligence services were kept in the dark, and even the soldiers involved in the operation did not know until the last instant that they would cross the border, said Gen. Valeriy Kondratiuk, a former head of Ukraine’s military and foreign intelligence service, who still maintains contacts among senior officials.

“We needed to seize the initiative and achieve something at the front,” he said.

Now, a month after the incursion, the FSB has released no information that might shed light on how the Ukrainians were able to cross the border successfully, and has given no sense of when Russian forces might be in a position to push them out.

The only information provided by the FSB’s press service has been about criminal cases opened against Western and Ukrainian journalists, who followed the troops onto Russian territory without following Russian customs procedures and getting passport checks.