The thunder of ferocious clashes unfolded at the nearby front line as the Ukrainian crew prepared to maneuver its American-made Bradley fighting vehicle out of camouflage and into the fire.

The commander of the team, a sergeant with the call sign Lawyer, nervously scanned the sky. “If we are seen, the KABs will come,” he said, referring to the one-ton bombs Russia has been using to target Ukraine’s most valuable armor and defenses.

What had started as a small Russian thrust into the tiny town of Ocheretyne was growing into a substantial breakthrough, threatening to unhinge the Ukrainian lines across a broad stretch of the eastern front. The crew’s mission was to help contain the breach: protect outmanned and outgunned infantry soldiers, evacuate the wounded and use the Bradley’s powerful 25-millimeter cannon against as many Russians as possible.

But the 28-ton vehicle was soon spotted. Mortars and rockets exploded all around, and the gunner was badly injured, said the commander, identified only by his call sign according to military protocol.

A combat assignment had turned into a mission to rescue his comrade. The gunner survived and is now recovering, Lawyer said a few days later as he recounted the attack. But the Russians gained territory and are continuing to try to press forward.

Ukraine is more vulnerable than at any time since the first harrowing weeks of the 2022 invasion, Ukrainian soldiers and commanders from a range of brigades interviewed in recent weeks said. Russia is trying to exploit this window of opportunity, stepping up its assaults across the east and now threatening to open a new front by attacking Ukrainian positions along the northern border outside the city of Kharkiv.

Months of delays in American assistance, a spiraling number of casualties and severe shortages of ammunition have taken a deep toll, evident in the exhausted expressions and weary voices of soldiers engaged in daily combat.

“Frankly speaking, I have fears,” said Lt. Col. Oleksandr Voloshyn, 57, the veteran tank battalion commander of the 59th Motorized Brigade. “Because if I will not have shells, if I will not have men, if I will not have equipment that my men can fight with…,” he said, trailing off. “That’s it.”

The sudden Russian advance through Ocheretyne, about nine miles northwest of Avdiivka, in late April, illustrates how even a small crack in the line can have cascading effects, as already-stretched platoons risk being flanked and encircled and other units race in to plug the breach.

“It’s like if you have an engine knock in your car, and you keep driving it,” Lt. Oleksandr Shyrshyn, 29, the deputy battalion commander for the 47th Mechanized Brigade, said. “The car works, but at some point, it will just stop. Then you’ll end up spending even more resources to restore it.”

“Similarly here, there are mistakes that do not seem critical,” he said. “But they’ve led to the need to stabilize the situation now. And it’s uncertain where that stabilization will occur.”

“Every event you did not predict can turn your situation completely upside down,” Lieutenant Shyrshyn said. “And this is what happened in Ocheretyne.”

The Ripple Effect

After the fall of Avdiivka to Russian forces in February, the small town of Ochertyne served as a Ukrainian military strong point along a highway. Most of the 3,000 residents had fled. Abandoned high-rise apartment blocks and other urban infrastructure provided good defensive positions and for two months, the situation remained relatively stable.

But then something went wrong.

The Russians appeared so suddenly on the battered streets around Ivan Vivsianyk’s home in late April that, at first glance, he mistook them for Ukrainian soldiers. When they asked him for his passport, the 88-year old knew the defense of Ocheretyne had collapsed.

“I thought that our soldiers would come and knock them out,” he said in an interview after making what he called a harrowing walk across the front line to escape. “But it didn’t happen.”

Three weeks later, what started as a small Russian advance has grown into a roughly 15-square-mile bulge that is complicating the defense of the Donetsk region.

Extending the bulge farther north could allow the Russians a chance to bypass some of the strongest Ukrainian fortifications in the east that have held for years. Russia can now also take a new line of attack aimed at Konstiantynivka, a town that is a logistical linchpin for Ukrainian forces.

The Kremlin’s bid to advance from one ruined village to the next has been captured in hours of combat footage shared by Ukrainian brigades at the front.

Russian infantry storm across mine-strewn fields on foot and use dirt bikes and dune buggies to try and outrace Ukrainian exploding drones. They attack in armored columns of varying sizes, with large assaults often led by tanks covered with massive metal sheds and equipped with sophisticated electronic warfare equipment to protect against drones. Western observers have dubbed them “turtle tanks.” The Ukrainians call them “wundervaflia,” which combines the German word for wonder with the Ukrainian word for waffle.

“We allow their infantry closer to us, which creates closer contact and direct firefights,” Lieutenant Shyrshyn said. “Therefore, our losses are increasing.”

The Russians are also paying a staggering price for every step forward. Some 899 Russian soldiers per day were killed or wounded in April, Britain’s military intelligence agency reported recently.

Despite throwing so many soldiers into the fight, the Russians took an area covering only about 30 square miles in April, according to military analysts. And capturing Ukraine’s last fortress cities in the Donbas — urban centers like Kramatorsk and Pokrovsk — would almost certainly involve long and bloody battles.

Still, the Russian advances in recent weeks in the east and northeast are starting to alter the geometry of the front in dangerous ways.

The Fraying Line

“Look at the map, where we are and where Ocheretyne is,” said Colonel Voloshyn, the tank battalion commander. He studied the terrain as he prepared to head out on a mission to target a house where 20 Russians were thought to be hiding. “I can now assume that they can simply bypass us on the left, on the right. They have tactical success, they have equipment, men, shells. So we can expect everything.”

The lack of dramatic shifts in the front for more than a year obscured the exhausting positional fighting needed to maintain that precarious balance. In a war where a battle over a single tree line can rage for weeks, the sudden Russian thrust into the area around Ocherytne was the most dangerous kind of problem — fast, deep and surprising.

There is a bitter debate over who was responsible for the failure to hold the line there.

The Deep State Telegram channel, which has close links to the Ukrainian army, accused the 115th Mechanized Brigade of leaving critical positions without orders, allowing the Russians to infiltrate and storm the settlement.

The brigade issued a furious denial, saying its soldiers were outnumbered by as much as 15 to one and held on as long as possible under withering bombardment.

“We want to emphasize that no regular unit of the 115th Brigade of the Armed Forces of Ukraine left or fled positions,” the brigade said. A special military commission has been established to determine what precisely what happened.

Soldiers familiar with the fight were hesitant to publicly criticize a neighboring brigade and said a host of issues — from poor communication to being vastly outgunned — all likely played a role.

Lieutenant Shyrshyn of the 47th, which held positions next to the 115th, would not speculate on what went wrong, but said the consequences were immediate: It was soon clear that the 47th would have to fall back or risk encirclement and catastrophic losses.

“The Russians sensed the weakness in that direction as they used the gaps to get in behind the Ukrainian soldiers,” he said. “Then we lost Ocheretyne, then Novobakhmutivka, then Soloviove.”

The Ukrainian high command does not like to surrender any territory, the lieutenant said, adding that “it is very complicated to argue with them and explain why it is not good to hold this position.”

Lieutenant Shyrshyn hoped the situation would improve with the arrival of Western weapons but until then, he said, “we will continue to die, we will continue to lose territories”

“The question is whether it will be at a slow pace and defensible,” he said. “Or at a fast one and senseless.”