The column of Bradley armored vehicles rumbled forward, filled with Ukrainian soldiers, bringing a new and potent American weapon to the war’s southern front.

But then one hit a mine. The explosion blew off one of the vehicle’s bulldozer-like tracks, immobilizing it. The entire Ukrainian column reversed direction, pulling back.

Three weeks into a counteroffensive critical to Ukraine’s prospects against Russia, its army is encountering an array of vexing challenges that complicate its plans, even as it wields sophisticated new Western-provided weapons. Not least is a vast swath of minefields protecting Russia’s defensive line, forming a killing field for Ukrainian troops advancing on the open steppe of the south.

“Everything is mined, everywhere,” said Lt. Ashot Arutiunian, the commander of a drone unit, who watched through a drone’s video link as the mine exploded under the Bradley and halted the column’s advance.

Over the weekend, a mutiny on Russian soil by mercenary forces raised hopes in Ukraine that its army might find the going a bit easier, even though the rebellion quickly died out.

But Ukrainians still face hurdles that differentiate this campaign from their swift push through the Kharkiv region in September and even from the more arduous offensive that recaptured Kherson in November.

The terrain in the southeast is mostly flat, open fields, in contrast to the rolling hills of the Donbas or the heavily forested north, depriving Ukraine’s troops of cover. The Russians have also been dug in for months in expansive trench lines, making uprooting them more difficult.

In addition, KA-52 Russian attack helicopters have been able to slip past air defenses, slowing Ukrainian movements while damaging or destroying Western-provided tanks and armored fighting vehicles.

And not only are the minefields bigger and more ubiquitous, but Russian troops have proved adept at replenishing some minefields cleared by Western-supplied equipment, a senior United States military official said.

Ukrainian forces in some locations along the front line are pausing to reassess which breaching and clearing tactics and techniques are working best, the official said.

The fierce resistance has taken a toll on Ukraine’s weaponry. The United States committed 113 Bradley fighting vehicles in March. At least 17 of them — more than 15 percent — have been damaged or destroyed in the fighting so far, the official said.

These obstacles have turned the early stages of the counteroffensive into a slow and bloody slog, limiting Ukraine’s forces to about four miles of territory gained in their farthest advance so far. That’s less than half the distance Ukraine needs to cross — threatened by mines and relentless Russian artillery bombardment — to reach Russia’s main defensive positions.

“They dug in, they mined, they are ready,” said Yevhen, a private with a paramilitary police unit who, like some other soldiers, insisted on being identified only his first name and rank. “It is difficult, but there is no other option.”

Despite the counteroffensive’s slow progress, Ukrainian officials say the main battles to breach Russian defenses are still ahead, and with the bulk of Ukraine’s force still kept in reserve, it is early to gauge success or failure, they contend.

Mr. Zelensky, while conceding that progress has been “slower than desired,” cautioned against what he portrayed as unrealistic expectations of a cinematic blitzkrieg through enemy lines.

“Some people believe this is a Hollywood movie and expect results now,” Mr. Zelensky said in an interview with the BBC this past week. “What’s at stake is people’s lives,” he said. “We will advance on the battlefield the way we deem best.”

In Washington, officials in the Biden administration are publicly urging patience even as they privately fret that the initial progress has been slow. One senior administration official called the results of the first couple of weeks “sobering,” adding, “They’re behind schedule.”

The senior U.S. military official also acknowledged the slower-than-hoped-for pace of operations but added that this was not unexpected given the extensive Russian defenses, and cautioned against drawing any broad conclusions based on the initial operations.

Both officials spoke on the condition of anonymity to discuss confidential government assessments.

Ukraine is seeking to split Russian-occupied territory in the south into two zones, cutting supply lines to the Crimean Peninsula and creating a springboard for further advances. To do so, it must push south about 60 miles from the former front line, where Ukraine halted Russia’s advances in March 2022, to the Sea of Azov.

Russia’s main defenses lie a dozen or so miles behind heavily defended territory. Those are the most difficult to cross.

Ukraine’s strategy has been to probe, striking at multiple sites to find a weak point in defenses. Russia, which has been preparing for the attack for months, is seeking to slow Ukrainian troops with mines, artillery, attack helicopters and counterattacks before they can find a gap and send troops flowing through it into occupied territory.

Success for Ukraine now hinges on how many tanks, armored vehicles and soldiers it can preserve before reaching the primary defensive line and in a battle to break through. Over the winter, Ukraine and Western allies trained and equipped about 40,000 soldiers for the attack.

“How much will they have left available at that point?” Michael Koffman, the director of Russia studies at CNA, a research institute in Virginia, said in a telephone interview. “A lot of what we see so far is inconclusive.”

At two of three points of attack, south of the town of Velyka Novosilka and the city of Zaporizhzhia, Ukraine has punched forward bit by bit and reclaimed a total of eight villages. At the third, south of the town of Orikhiv, where the Bradley hit a mine, the assault has appeared to stall in the fields.

Curiously, Ukraine has advanced in the two locations where troops were provided fewer new Western weapons, and stalled where the most sophisticated new weapons — American Bradleys and German Leopard 2 tanks — were deployed.

It’s not clear if that is because Western weaponry was intentionally deployed in areas where there were stiffer Russian defenses.

Local factors, soldiers fighting in this area said, could explain the slower progress where Western weapons were deployed. The nearest villages, useful for finding abandoned houses, basements and root cellars for cover, are farther from the front line than at other sites.

And out in the open fields, the artillery bombardments have been so intensive, said a drone pilot who flies over the area regularly, that the battlefield “looks like Swiss cheese.”

Even as they temper expectations, Ukrainian officials have insisted the battle is on track. General Valery Zaluzhny, the commander of Ukraine’s army, published a video this week showing him perusing a large map and saying the fight was going “according to plan.”

Out in the expanse of farm fields in southern Ukraine, soldiers fighting on the front or assisting in medical evacuations said they understood the strategy of probing attacks, and that some would succeed and others would not. But they said the Russian defenses were formidable and progress is slow.

Lieutenant Yaroslav, a medic who has been evacuating wounded from the fighting, said the wounded described harrowing battles. “Given what the guys are saying, it’s not going as well as they show on TV,” he said.

On one axis of attack, Ukraine has advanced more quickly than anticipated. Soldiers fighting south of the city of Zaporizhzhia said they had been ordered to advance with no Western heavy weaponry. After reclaiming the village of Lobkove, the soldiers found they were close enough to the next village, Piatykhatky, to hear its dogs barking. It would not be hard to slip over to reclaim it, a soldier said, and this was done last week.

At a Ukrainian gun line, the artillery officer, a lieutenant named Arseniy, rattled off the types of rounds Ukraine fires: shrapnel for infantry in the open, a detonator with a delay for burrowing into and blowing up bunkers, and shells filled with leaflets explaining how to surrender — part of a Ukrainian psychological warfare operation to chip away at Russian morale.

On a recent dawn, after a rainstorm had blown over the night before, the gunners prepared a Soviet-legacy howitzer of a type nicknamed the Carnation. The barrel swiveled. “Fire!” a soldier yelled. The gun boomed. Leaves fluttered down from nearby trees.

A few minutes later, the artillery team was sent by an intelligence unit an intercept of Russian walkie-talkie communications. “Probably two dead,” a Russian commander said. The soldiers were in a buoyant mood.

“It’s our usual working day to destroy as much as possible,” Arseniy said.

Of the counteroffensive, which he sees through the ebb and flow of orders to fire the gun, he said, “I think it is going to plan,” but then added, “Even if things go not according to plan, that is also in our plan.”

The once sleepy country roads, lined with tall green grass and wildflowers, are now clogged with ambulances leaving the front, their lights flashing. Tracked vehicles rumble along, and pickup trucks spray painted with makeshift camouflage, the main transport for soldiers, bounce over the ruts.

As twilight faded into night, and swallows swooped and screeched over the fields, a Ukrainian drone surveillance unit attached to the 47th Mechanized Brigade went to work.

These first hours of night are prime time for hunting Russian tanks with infrared cameras, as the bulky metal armor, warmed in the sun through the day, all but glows in the dark.

“Sunset is our golden time,” said the commander, Lt. Arutiunian. The soldiers spot tanks, then call in coordinates to an artillery team.

“We are testing their defenses” said Lt. Arutiunian. “I would not call it a full-scale attack yet,” he said. “We are probing.”

Andrew E. Kramer reported from Zaporizhzhia, Ukraine, and Eric Schmitt from Washington.

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