Has a Russian collapse along its eastern line begun? When it became clear that Russia’s attempt to decapitate Ukraine’s government by taking Kyiv failed, analysts noted that the clock had started on Russian stamina for its invasion. Some predicted that Vladimir Putin would have to either achieve victory in some overwhelming fashion as early as September or face ruin as the invasion exhausted its resources.

Well, September has arrived, and so has a multi-front Ukrainian counter-offensive. And it appears that the collapse may have started in Kupyansk, a key strategic position that could put the Russian eastern line in danger:

Ukrainian forces seized most of a strategically vital city in northeastern Ukraine on Saturday, cutting the main supply line to thousands of Russian troops near the eastern city of Izyum and marking the biggest strategic gain Ukraine has made since the start of an offensive this week.

Photos from Russian and Ukrainian channels on Telegram showed Ukrainian soldiers holding the country’s flag in front of the city hall in Kupyansk, and Kremlin-loyal Russian military correspondents said Moscow’s forces had pulled back across the Oskil River to the eastern part of the city.

Ukraine’s control over the critical infrastructure of the city and the success of this week’s advance signal to Western backers the effectiveness of weapons the U.S. and Europe has given to Kyiv.

The seizure of this city may have doomed thousands of Russian troops in positions gained in late spring:

The Kupyansk rail and road hub located in the western half of the city was the last artery connecting Russia with thousands of troops on territory that represented the bulk of Russia’s gains in May and June. Ukraine’s control of the road network also threatens Russia’s hold on Izyum, a city Moscow had planned to use to launch further attacks on Ukrainian-controlled parts of the Donetsk region in the country’s east.

The U.K. Defense Ministry said Saturday that Izyum was becoming increasingly isolated. Analysts say Ukraine is aiming to encircle a pocket of Russian troops around Izyum east of the Oskil River.

“Izyum will soon be ours,” said a Ukrainian commander fighting near the city.

The new offensive has captured a lot more than just Kupyansk (also spelled Kupiansk in some reports). Ukraine claims that it has reclaimed more than 2500 square kilometers in and around Kharkiv Oblast since the beginning of the month. ISW reports that significant numbers of POWs have been transported out of the area.

The Kremlin has been uncharacteristically silent about these developments, but there’s a sense of panic now. They’re shifting the same units back to the Kharkiv theater that they shipped to Kherson to deal with the counteroffensive there, and that’s getting notice by Russian milbloggers who have been reliable in their propaganda until recently:

Ukrainian forces may collapse Russian positions around Izyum if they sever Russian ground lines of communication (GLOCs) north and south of Izyum. Ukrainian forces continued to advance on Kupyansk and towards Izyum on September 9, and are undertaking measures to isolate the Russian Izyum grouping of forces. If Ukrainians are successful in severing the Russian GLOCs, then they will have an opportunity to create a cauldron around Izyum and collapse a major portion of the Russian positions in northeastern Ukraine.

The Kremlin is rushing resources to the Kharkiv City-Izyum line in an attempt to halt Ukrainian advances after Ukrainian forces achieved remarkable operational surprise. The Russian Ministry of Defense (MoD) and Kremlin wires published footage of Russian military convoys reportedly en route to reinforce Kupyansk, Izyum, and the general Kharkiv direction but did not acknowledge Ukrainian successes in the area.[3] While Russian milbloggers largely welcomed the reports of reinforcements, some criticized the Kremlin for first relocating units away from the Kharkiv City-Izyum line, only to deploy them again to the same location.[4] Russian forces have been redeploying out of southern Kharkiv Oblast to reinforce Donetsk Oblast and the Southern Axis to address the threat of a Ukrainian counteroffensive in Kherson Oblast and to resume offensive operations west of Donetsk City for several weeks.[5]

And ISW made this prescient call late yesterday:

The successful Ukrainian counteroffensive is upending the Kremlin’s effort to make Izyum an economy of force area. Some milbloggers also noted that September 10 will be a decisive day if Russians are unable to generate reserves and capable command in time.

The loss of the rail head in Kupyansk would make today a critical moment in the war indeed. And that would explain what little Russia is saying about its Kharkiv line, which is an acknowledgment of defeat:

Russia’s Defense Ministry said Saturday that it is pulling back forces from two areas in Ukraine’s Kharkiv region where a Ukrainian counter offensive has made significant advances in the past week.

Defense Ministry spokesman Igor Konashenkov said the troops would be regrouped from the Balakliya and Izyum areas to the Donetsk region. Izyum was a major base for Russian forces in the Kharkiv region.

Konashenkov said the move is being made “in order to achieve the stated goals of the special military operation to liberate Donbas,’” one of the eastern Ukraine regions that Russia has declared sovereign.

The claim of pullback to concentrate on Donetsk is similiar to the justification Russia gave for pulling back its forces from the Kyiv region earlier this year.

Remember what excuse Moscow used for that “regrouping”? It was to focus its attention on the south of Ukraine, especially Kharkiv and Kherson — the two points between which Russia has flip-flopped resources to avoid getting pushed off its lines. If all they’re able to do now is “regroup,” that makes it look like Russia’s playing a shell game with its exhausted resources in Ukraine.

But it might be even worse for Russia, CNN reported later in the morning. Ukraine now claims that its troops may have penetrated as far as the outskirts of Izyum itself:

Kyiv says its troops have entered the key city of Izium as Russian forces flee from a rapid counteroffensive. https://t.co/g97mMsj7Wh

— CNN Breaking News (@cnnbrk) September 10, 2022

The Associated Press notes the tactical surprise Ukrainians achieved in this counter-offensive still remains in effect. British analysts marveled at the “economy of force” used by Russia to defend this vital strategic link:

Earlier Saturday, the British Defense Ministry told reporters it believed the Ukrainians had advanced as much as 50 kilometers (30 miles) south of Kharkiv, and described Russian forces around Izyum as “increasingly isolated.”

“Russian forces were likely taken by surprise. The sector was only lightly held and Ukrainian units have captured or surrounded several towns,” the British military said, adding that the loss of Kupiansk would greatly affect Russian supply lines in the area.

Ukraine has kept up the pressure in the south as well:

The fighting in eastern Ukraine comes amid an ongoing offensive around Kherson in the south. Analysts suggest Russia may have taken soldiers from the east to reinforce around Kherson, offering the Ukrainians the opportunity to strike a weakened front line.

Ukrainian Defense Minister Oleksii Reznikov told the television channel Ukraina that the Russians had no food or fuel for their troops in the area as Kyiv had cut off their supply lines.

“It will be like an avalanche,” he said, predicting a Russian fallback. “One line of defense will shake and it will fall.”

That matches up pretty well with the post-Kyiv assessment of Russian capabilities. The first weeks of the war showed Russia’s deficits on leadership, tactics, and morale. The next few weeks showed how poorly their materiel performed in the field, but they still had the tactical advantages of quantity. Those are now being rapidly exhausted, leaving Russia with nothing except an overstretched line facing ferocious and well-armed opponents looking to regain their own land.

One line of defense for Russia is more than just shaking. And it looks like it may fall right on schedule.

Update: If this is true, then the Russians are in real trouble:

Ukrainian forces entered the key Russian military stronghold of Izium on Saturday, continuing their rapid advance across the northeast and igniting a dramatic new phase in the more than six-month war.

“Izium was liberated today,” the city’s mayor, Valeriy Marchenko, said in an interview. While he was not yet in the city himself, he said that he was in contact with the police and that emergency services were working to clear it of possible hazards before residents could return.

Russia’s Ministry of Defense — which a day earlier had said that it was moving to reinforce its defensive positions in the region — confirmed on Saturday that it had pulled its forces out of Izium, six months after its forces laid siege to and then seized the city. In a statement, it presented the retreat as a preplanned move, intended to strengthen its efforts in the east where its army has been bogged down for weeks.

This NYT report goes farther than CNN’s earlier flash update, which only implied that Ukrainian troops had gotten close to Izyum. If Ukraine has already taken it back, the collapse may be unstoppable.