Kharkiv, Ukraine

We are in the basement of the Kharkiv Opera House. It’s a good shelter, designed in the Soviet era to withstand a nuclear attack. It’s here that inhabitants of the country’s second-largest city come when the bombardments are incessant or when a French writer arrives to screen his film on the Ukrainian resistance.

The attendees include civilians and combatants, amputees on crutches, members of the legendary Kraken commandos led by a young Englishwoman who just stepped out of a novel by Graham Greene, widows of soldiers and mothers of deported children, an Orthodox priest and a rabbi. The film is titled “Glory to the Heroes”—in Ukrainian, “Heroyam Slava,” the standard response to the call “Slava Ukraini.” Ukraine’s heroes have come to watch on a makeshift screen a celebration of their courage.

It is July 13, the eve of Bastille Day. France is the capital of liberty, Kharkiv Mayor Ihor Terekhov, a host of the event, observes. Kharkiv, I add in my presentation, is the front line of the battle against tyranny. France and Ukraine. André Malraux’s observation that “France is never greater than when it is great for all people” has haunted me since I embarked on my first overseas reporting trips a half-century ago. It resonates here.

My presence in Kharkiv wasn’t announced until the last minute. By the time the screening ended, the news is buzzing on social networks. Groups of “Russian avengers” are vying for the honor (and the bounty) promised to those who would make me regret having come.

Once night falls, we have to cross the deserted city, with not a light shining, to meet Slava Vakarchuk, the Ukrainian musician who composed the scores for my film. We had left him near the front that afternoon, where he was performing for the troops. Now he’s having dinner at an obscure location on the edge of the city with Igor Obolenskyi, commander of the Khartia brigade, an elite unit of the Ukrainian National Guard. Also present is Serhiy Zhadan, 49, a poet and writer, who has just enlisted in the unit.

I immediately take this newcomer into the holy family of combatant writers. I admire his sad, hard-eyed look in recounting his decision to move beyond words and take action. He has the air of a talker who has gone silent. And although his neck is shaved, he still has the look he had when he was slamming his poems, accompanied by the drums, synthesizers and guitars of his friends in the Yara Arts Group on the rock stages of Lviv and Kharkiv.

We talk of Pierre Choderlos de Laclos and Lord Byron, George Orwell in Catalonia and Curzio Malaparte in Italy. After consulting Mr. Obolenskyi, his chief, he offers a suggestion: Why not come along with us tomorrow on an operation?

We meet at the last gasoline station before the Russian border, where Mr. Obolenskyi is waiting for us with four elite soldiers from Khartia. Mr. Zhadan is there, too, of course, wearing a bulletproof vest emblazoned with “Radio Rocks.”

We drive 10 kilometers north on the dirt roads that cut through the heath, passing through villages that Russian glide bombs have reduced to ruins. Arriving at Lyptsi, the epicenter of the fighting, we leave the cars in the brush and continue briskly on foot until we reach a farm building, of which nothing remains but the entrance to a stairwell leading down into a cellar. In the bunker, five more men sit on camp beds, their eyes burning from lack of sleep. There ensues a surrealistic discussion about the French elections and the preceding day’s assassination attempt on Donald Trump. And then the commotion of battle.

“Go!” Mr. Obolenskyi shouts. His men rise in a rattle of breeches and loaders and rush upstairs. We run for a kilometer in an open field, stumbling over rubble mixed with chunks of mud dried by the heat. We stop twice to catch our breath in the shelter of a still-standing wall that resembles a calvary. We arrive, still running and out of breath, at a 155mm cannon hidden in a grove.

The unit gets into firing position, covering the gunners. A soldier plugs in a GPS device and scans the sky. “Fire!” Mr. Obolenskyi orders in a different sort of voice. “Fire!” he shouts again, louder now. I watch two streams of flame leave the gun. But under the thick earmuffs of the helmet Mr. Zhadan gave me at the last minute, I first hear the sound of my breath and of the blood pulsing too hard and fast in my temples. The order is given to fall back without delay, following a slightly different path, first to the cars and then to another cellar, where we receive a very unusual debriefing.

I can’t give the exact location. But imagine a wall of screens being watched with the utmost concentration by a small group of men who appear simultaneously to be soldiers (wearing fatigues and tattoos) and geeks making mysterious notations in old-fashioned school notebooks.

“I’ll explain everything about what you’ve just experienced,” Mr. Obolenskyi says. “This is Boris, who, when we were discussing Trump and the French elections, was telling me through my earpiece, ‘The sky is dirty.’ And this is Sergiy, who, when he saw no more suspicious drones in the zone, updated the report to ‘The sky is clean’ and gave us the go. There, on that other screen, you have our drones, which, while we were running, were following the target (a Russian unit preparing an attack) that I fired on and, on the second try, completely eliminated. And, on this same screen, you see these blue dots?”

He has the operator enlarge the image. “These are robots that we deploy in the no man’s land and that, if a Russian vampire drone had come in despite everything, would have scrambled its systems and skewed its aim. That’s our brigade. Ultramodern. Ultra-economical with the human resources who are Ukraine’s treasure and who we don’t send into action unless we can protect them. How could Putin not hate this fine, knowledgeable town, so up to date on every technology?

“And take a look at this.” He points to another screen, on which I can make out shadows that I understand to be Russian military infrastructure across the border. “A few weeks ago our allies finally authorized us to strike deep within enemy territory, while respecting the NATO charter!” He bursts out laughing. “There you have it. Our unit is called Khartia, because we scrupulously respect the charter. That’s it in a nutshell.”

Kupiansk, in the southeast, is another of the hot spots along the front where the Russians are trying to punch through. I visited the city nearly two years ago after the Ukrainians had retaken it, and there reigned an atmosphere of a liberated Naples on a smaller scale. At that time, babushkas were beginning to return and to proclaim, from the threshold of their destroyed houses, the return of “real Ukrainian borscht.”

Today the city is empty. The streets have become ghostly again, the targets of bombardments. The Ukrainian forces stationed at the entrance to the western suburbs make one last effort to loosen the stranglehold this evening in an artillery duel with Russian forces firing from the valley 5 kilometers away.

“You see why we are in such dire need of long-range weapons?” asks Gen. Artem Bogomolov, commander of the region’s defense forces. “Listen . . . count them. . . . You can hear the difference between outgoing and incoming. . . . As we speak, you have an average of eight incoming for every outgoing. Eight missiles falling on us for every one going out.”

We are on the highest piece of ground, which, like a giant promontory, overlooks both Kupiansk and the enemy’s position. And because the light is fading we can see the long trails of the shells streaking the sky above our heads.

I muse that the last time I saw Gen. Bogomolov was in the martyred city of Bakhmut, where his troops held out for a year while the city was gradually destroyed. I notice in his jovial, round centurion’s face the sudden appearance of a fevered, almost horrified, look that he hadn’t had before. And a presentiment crosses my mind: What if lovely Kupiansk were to become the next Bakhmut? What if the barbarians facing us had decided to reduce it, too, to a pile of ruins and ash?

Denys Prokopenko, commander of the Azov Brigade, is stingy with words. We find him in a secret camp in the woods to which we were brought in a state-of-the-art armored personnel carrier, made in Ukraine and equipped with a drone detector that had us stop three times on the way. He has the same ruddy cheeks, the same bright smile, and the same blue eyes of a bird of prey that impressed me at our first meeting, in Mariupol in 2020.

In his command tent, he recounts the siege of Azovstal, that underground Masada where a thousand ragged people, cut off from the world, resisted the Russians from March to May 2022 with the sole and successful goal of delaying the fall of Mariupol.

He recalls the debates on that last fateful night, when President Volodymyr Zelensky gave the order to surrender and more than a few of his companions believed that it would be better to die there as heroes, arms at the ready, than to rot in a Russian prison.

He recounts the Russian jails, the torments he endured there for four months (until his release on Sept. 21, 2022), torments on which his pride doesn’t allow him to dwell. He says that during moments when all seemed lost, the only thing that gave him the strength to hold on was the idea that he had to survive so that one day he could share the incalculable stories of bravery and resistance during that epic siege.

I am aware of the bad reputation Vladimir Putin’s propaganda has tried to give Azov. Ten times since this interminable war began, I have had the opportunity to investigate the history of this battalion that grew into a regiment and finally a brigade while purging itself of the ultranationalists and antisemites who were part of it at the outset.

During this vigil, on the eve of an offensive where everything indicates that the brigade will bear the brunt, I extend to its commander—as we walk a patch of ground where it is so hot that the steps of the men around us raise swirls of dust resembling clouds of smoke—my friendship and respect.

Like Mr. Prokopenko, Gen. Oleksandr Syrskyi speaks little. A self-effacing hero of the battle of Kyiv, then liberator of Kharkiv, and now commander in chief of the Ukrainian armed forces (he succeeded Valerii Zaluzhnyi, whom Mr. Zelensky fired in February), Gen. Syrskyi shuns journalists and spends all of his time in the field with his troops.

We are on one of his bases further north, deep in the forest, where we were led amid great mystery, following routes that all looked the same, routes in which our 4x4s bogged down and lost their way.

It is a camp of tents hidden in the trees and furnished with benches decorated with pious images in gaudy colors. A thousand men are there. They keep busy, but they’re feverish. They seem impatient to go into combat, though they are tired. They come to attention when Gen. Syrskyi appears. Sometimes they let forth a resounding “Slava Ukraini!” when he passes, to which he responds with a sober “Heroyam slava.”

In the faces around me, I see a new weariness that I never detected in any of my previous visits. “Everyone is getting tired,” the general tells me, as if he were reading my mind. He stops and looks me in the eye. “We’re up against terrorists who are using a steamroller strategy, sending waves of human meat to the slaughterhouse. Can you pass on a message?” He pauses and takes me aside with my friend Serge Osipenko to translate. We have come to a large clearing resembling a ruined meadow, where the heat is once again overpowering.

“We are grateful to President Macron,” he says. “The steadiness of his support, as well as his willingness to send us instructors, is appreciated. But we need more in order to respond to this barbarism and, through our response, to enable Europe to stand up to it. Planes. Cannons. Notably your Caesar howitzers, which are among the best in the world but which we don’t have enough of.”

France riding to the rescue of Ukraine? Europe taking over from an America that the Ukrainians know may soon be led again by Mr. Trump? That may be the message.

We’ll soon leave for Odessa, where we want to see the Cathedral of the Transfiguration, which was shelled last year and has still not been rebuilt. Gen. Syrskyi, like a crouching wolf waiting to pounce, disappears into the brush, under a sky full of suddenly threatening clouds, into his realm of brave fighters who hold in their hands a piece of our destiny. He has a plan. He can win. For that to happen, it is necessary and sufficient that Europe (and perhaps America) give him the means. Will they have the intelligence to do that? The courage? May God make it so.

Mr. Lévy is author of “Israel Alone,” forthcoming in English Sept. 10, and author and director of the documentary “Glory to the Heroes.” This article was translated from French by Steven B. Kennedy.