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.

Russian 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.