Russians Poured Over Ukraine’s Border. There Was Little to Stop Them.

Russian troops punched across Ukraine’s northern border with such speed and force last week that Ukraine’s meager fortifications offered almost no obstacle. Some Ukrainian soldiers, caught totally by surprise, fell back from their positions, and villages that had been liberated nearly two years earlier suddenly came under relentless shelling, forcing hundreds to flee in scenes reminiscent of the early days of the war.

“They are erasing streets,” said Tetiana Novikova, 55, a retired factory worker who said she barely escaped with her life on Friday when her village of Vovchansk came under withering fire from Russian forces. As she fled the village where she had spent her whole life, she said, not a single Ukrainian soldier was in sight.

The stunning incursion into the Kharkiv Region lays bare the challenges facing Ukraine’s weary and thinly stretched forces as Russia ramps up its summer offensive. The Russian troops pouring over the border enjoyed a huge advantage in artillery shells and employed air power, including fighter jets and heavy glide bombs, to disastrous effect, unhindered by depleted Ukrainian air defenses.

Once over the border, the Russian soldiers easily pushed past fortifications — like trenches, land mines and tank barriers — some of which, Ukrainian troops said, were insufficient or sloppily constructed.

But the biggest challenge for Ukrainian forces is people. Ground down over more than two years of war, Ukraine’s military is struggling to come up with enough soldiers to effectively defend the 600-mile front line, even as Russian forces have swelled with thousands of newly mobilized troops.

As the scale of the Russian push became clear over the weekend, Ukraine’s military scrambled to divert troops from other areas of the front, rather than deploying reserves. The reason, according to Ukrainian officials: There are few reserves to deploy.

Ukrainian military officials, speaking on the condition of anonymity to discuss sensitive military details, said the situation in the Kharkiv Region was critical, but under control. On Saturday, Ukrainian forces appeared to have slowed the Russian advance, though vicious fighting was reported along a ribbon of territory five miles from the Russian border.

As of Saturday, nearly 10,000 residents of the Kharkiv Region had fled the fighting, according to the regional governor, and residents reported that whole villages had been wiped out. As Russian troops advance, there are fears that for the first time in nearly two years, they could come within artillery range of Kharkiv, Ukraine’s second largest city.

The Russians “know what they are doing,” said a Ukrainian commander, speaking on the condition of anonymity to discuss details of the incursion. He added that he was sorry for civilians who thought they were safe.

For months, Russia has been building up troops along Ukraine’s northern border, with 50,000 deployed in the area around the Kharkiv and Sumy Regions, according to Kostiantyn Mashovets, a Ukrainian military analyst. Russia’s foreign minister, Sergey V. Lavrov, signaled Moscow’s intentions when he said that Kharkiv “plays an important role” in President Vladimir V. Putin’s stated desire to create a “sanitary zone” along the Russian border.

Ukraine’s top officials appeared to be taking the danger seriously, with President Volodymyr Zelenksy making a highly choreographed visit to the fortifications around Kharkiv on April 9.

“We have to be prepared,” Mr. Zelenksy said. “And Russians must see that we are ready to defend ourselves. And our people must understand that Ukraine is prepared in case the enemy tries to attack.”

Those preparations did little to blunt the Russian attack. Part of the problem lies with restrictions on the use of sophisticated American weaponry. Though Ukrainian forces could see the buildup on the border, a White House prohibition on using high-precision American weapons, like HIMARS multiple rocket launchers, against targets on Russian territory prevented Ukraine from attacking them.

Then there is the disadvantage Ukraine has operated under since the beginning of the war: They are facing a much bigger country with a sizable manpower advantage and a large arsenal of weapons that is constantly replenished by a defense industry operating with a record budget.

Some Ukrainian officials said that fortifying areas close to the border was nearly impossible because of Russian shelling. But, they added, stronger defensive lines, constructed farther from the frontier, have so far held up under the Russian assault

Iryna Sykhina, 42, of Lyptsi, a town about 10 miles north of Kharkiv, said she understood something was different, and wrong, in the early hours of last Friday, when her village came under relentless Russian shelling. “They were hitting the entire village at once, not just from time to time like before,” she said in a phone interview.

Ms. Sykhina said that she had seen concrete blocks and machinery being moved along a road in front of her house, in what she believed were preparations for fortification.

“But in fact, from what I know, nothing was built,” Ms. Sykhina said.

Once the Russians opened their attack, some points along the Ukrainian lines buckled and troops fled amid the heavy bombardments, said Denys Yaroslavsky, a lieutenant with the 57th Brigade.

“There are many more questions for those who were responsible for building fortifications on the first line, those who were supposed to mine it and strengthen it,” he said.

The mayor of Vovchansk, Tamaz Gambarashvili, insisted in an interview that his town was prepared for an incursion. “I have been in this town all the time and I can say we were ready for Russians to come and fortifications were done,” he said in an interview this past week.

The fortifications, he said, were not constructed of concrete “because Russia was constantly shelling everything we were building,” but he added, “the ones done by hand were prepared to the maximum.”

Officials and military analysts say that Russia probably has insufficient forces to take the city of Kharkiv. After nearly two years of relative tranquillity, long-distance missile attacks now occur daily, killing and wounding civilians. Russia appears to be relying on its numerical superiority to stretch Ukrainian forces to the breaking point.

Russia now has about 510,000 troops in the fight, according to an analysis published this week by Royal United Services Institute, a British think tank. Those numbers are sufficient to launch attacks along the full length of the front line, keeping Ukrainian forces constantly off balance as they struggle to respond to multiple incursions.

“Russia’s aim is not to achieve a grand breakthrough, but rather to convince Ukraine that it can keep up an inexorable advance, kilometer by kilometer, along the front,” the analysis said.

New weaponry set to arrive as part of the Biden administration’s $61 billion arms package, including artillery shells and air defense ammunition, should help alleviate some of the strain on Ukraine’s forces, as should a new mobilization drive by Ukraine’s government, which lowered the age of conscription to 25.

The question now, as the summer fighting season begins in earnest, is whether it will be enough to halt Russia’s momentum.

For residents along Ukraine’s northern border the strain is difficult to bear. Much of the region was quickly occupied in the opening days of Russia’s invasion, which began in February 2022. But Ukrainian forces pushed them out six months later in a stunning offensive operation that now seems like a high point in the war.

Villages and settlements in the region have long been subjected to intermittent Russian shelling, and many residents have fled. Still, those who stayed said they had been caught by surprise by the assault and the Russian troops’ quick advance.

“People had been preparing for the summer, tending their gardens, caring for their livestock,” Krystyna Havran, a member of the village council of Lyptsi, said in an interview. “No one imagined that there would be an offensive.”

Marc Santora contributed reporting.