The huge troop-transport helicopter dates from the Soviet era. It flies just above the tree line, nose down, to avoid Russian radar. After two hours of flying over smooth ground, frozen lakes and ruined villages in the dark winter night, we arrive here at the headquarters of the Navy Guard, where we meet the officers who for six years have led the fight against pro-Russian separatists in the Donbass region. 

I didn’t wait for the briefing. The nearly deserted fish market in the center of town told the story. So did the empty shops on Lenin Avenue and the enormous blast furnaces of the Azovstal factory, running at half power and emitting thin clouds of dirty black smoke. This shutdown of a key industrial city wasn’t caused by a pandemic. The separatists, unable to take control of the city, imposed a blockade.

In Shyrokyne, a seaside resort half an hour’s drive east, all that remains of its 2,000 residents are a couple, Maxime and Tatiana, former innkeepers who’ve come in under the protection of a national guard unit to lay flowers at his father’s grave. The elegant houses that lined Shapotika and Pushkin streets are reduced to piles of rubble. Lt. Martha Shturma, our interpreter, says this was an ordinary resort with no strategic importance. Its remains to tell a story of gratuitous destruction: the church with the collapsed roof; the clinic reduced to concrete pillars and mined; the school pulverized by heavy artillery, where we find a section of blackboard, half-burned notebooks and a backpack, miraculously spared.

The war isn’t over, as I see 45 miles north in Novotroitske, where the 10th Battalion of the Mountain Assault Brigade is positioned. An armored ambulance covered with red crosses carries us on a rough track for an hour to the first outposts. Not far from here, a soldier was killed and another wounded this morning at 7:15.

Accompanied by Gen. Viktor Ganushchak and a special forces unit, we weave our way through an endless network of trenches, all zig-zags and corners. Some are deep, like tunnels or caves, and shored up with boards and logs. Others are open-air but hidden behind a curtain of artificial gray. Sentinels sit every 50 yards, sometimes in a bunker heated by a coal stove, sometimes behind a lookout station lined with old straw. An officer proudly explains that these disciplined sentinels will prevent any more breakthroughs like those that occurred during the lightning pro-Russian offensives of 2014 and 2015. I dare not tell him the scene reminds me of Verdun.

Further north is Krasnohorivka, where the soldiers were hit this morning. We covered the last few miles by spacing our vehicles well apart, since here the enemy is only a few hundred yards away. Ukrainian officers always say “enemy,” not “separatists” or “pro-Russian.” They understand themselves to be fighting the Russians themselves, not the pro-Russian militiamen who are firing at them. 

“Look,” Col. Maxime Marchenko says, pointing at what’s left of a Grad missile. “Only the Kremlin has weapons like that. And come look at this . . .” We climb to the seventh floor of a government office building converted into a field headquarters and topped with a watchtower. Using binoculars to see through an opening in a wall of sandbags, we make out the suburbs of Donetsk, an immense city that is now the capital of the separatist-proclaimed People’s Republic of Donetsk.

Donetsk was known as Stalin (1924-29) and Stalino (1929-61). Today it’s a Soviet Jurassic Park, with its concrete housing blocks, downtown factories, slag heaps and the metal carcasses of its destroyed airport. In the foreground stands a stationary convoy of Gvozdika self-propelled howitzers, the type Moscow used in the second Chechen war. It’s pretty hard to imagine how they could have come from anywhere but Moscow’s arsenals. 

In the Myrolyubivka zone, farther north and not as close to the front, we come on a firing range with three 155mm cannons and 20 or so young Ukrainian artillery men busying themselves around the mouths of the guns. The commanding officer says it’s just a drill. But he can’t contain his pride at the machinery, the skillful men operating it, and the honor they feel in obeying the orders of their civilian commander, President Volodymyr Zelensky, and in observing the cease-fire laid down in the Minsk Protocol in 2014. 

I couldn’t help thinking back five years to the battalion I saw on the outskirts of Kramatorsk, not far from here, which had just been savagely bombarded. That unit was short of equipment; the soldiers’ faces deathly pallid from lack of sleep. One even stood on crutches as they met with then-President Petro Poroshenko. Ukrainian defenses have come a long way.

Pisky, further north still, has been completely destroyed—and mined. We had to enter it on foot, single file, behind the patrol that came out to meet us. Not one building had escaped destruction. Boards had been hung in the shape of crosses over one bombed house’s empty windows. Streets looked like vacant lots, with dead grass competing with fresh snow. Water, power and sewer lines had all ceased to function. Of the 2,000 or so who once lived here, only three families remain, holed up in their basements—though the patrol’s leader hasn’t seen them for weeks.

We encounter a starving dog licking the edge of a stone well. Another lies dead on a pile of rubble, paws stiff and eyes glassy. Pisky is a ghost town; even the beasts are specters. Nothing on the trip will haunt me more than this gutted, lifeless landscape through which we move among pale, bloated shadows. 

I don’t know what came over me the next day. One more cadaver won’t make a difference in a war that has already claimed 13,000 lives, but I awoke with an irrepressible need to learn more about the wounded soldier and his buddy who’d been killed at 7:15. So we headed to the field hospital in Pokrovsk. The fallen soldier, Yevhen Shchurenko, is in the morgue, his head blown off, his body dressed in a fresh uniform.

The wounded man shares a room with five other patients, both civilians and soldiers. He is silent at first and has the feverish, absent look of those for whom nothing matters except to feel a little less pain. But then he changes his mind. He carefully pulls back the sheet to show us the bandages on his abdomen and thigh. He tells us two things in a weak yet steady voice: He was hit by shrapnel as he jumped into the trench to man his post after morning duties. And he had placed too much confidence in the European patrols that are supposed to monitor the cease-fire.

This reminds me that at 10:30 a.m. on the same day the rocket hit, we’d seen two white cars from the Organization for Security and Cooperation in Europe pull up, not far from the rocket’s landing spot. A Ukrainian officer, addressing no one in particular, sarcastically observed that these “clowns,” who invariably show up after the fact, were supposed to have been there since dawn. 

Stanytsia Luhanska is at the north end of the frontline, a checkpoint the belligerents set up to allow Ukrainians to cross from one zone to the other. It is a corridor bisected by a metal fence with a sort of customs station on either side—separatists to the east, loyalists to the west. Except for one detail that is hard to miss. As we watch, barely anyone moves into the separatist zone, which here is styled the People’s Republic of Luhansk.

In the other direction stand endless lines of babushkas with shopping bags, wizened seniors pushed along in wheelchairs, and young people there since dawn. It turns out that Ukraine, which views the inhabitants of Luhansk and Donetsk as hostages of Vladimir Putin and the separatists, still honors their rights as Ukrainian citizens, and continues to pay their pensions. They have to cross over to withdraw their money from ATMs in loyalist Ukraine. The stores being empty on the separatist side, they spend their modest pensions in Ukraine on the staples they need for the month.

In Zolote, which borders Luhansk, we encounter more trenches. Cruder than those of Novotroitske, they consist of little more than boards laid onto the dark ground. Yet they’re more striking because of the huge dogs that guard the entrances, like Cerberus at the gates of hell, and especially because of the overequipped Rambos, their faces darkened by dirt or tattoos or hoods, standing guard 10 yards apart. 

They seem like professional soldiers lying in wait. Might they be members of the Azov and Aidar battalions, famous for their bravery and infamous for harboring ultranationalists and even neo-Nazis? Apparently not. I met with Azov’s commander, Denis Prokopenko ; his men were training in Mariupol. I saw Aidar’s commander, Oleksandr Yakovenko, but in Kyiv, where he told me that his 760 men were “on rotation.” 

These camouflaged troops—who seem raring to go, younger than those I have seen up to now, and noticeably more composed, are close-combat pros. I am told that they have just been inspected by President Zelensky. They man a defensive position, but it’s easy to imagine, as at the firing range in Myroliubovka, that it could be quickly converted into an attack base.

In Kyiv I meet Mr. Zelensky, sitting in the same kitschy office where I often sat when Mr. Poroshenko was president. Around the faux marble table, he takes the same seat his predecessor always occupied. And he invites my collaborator Gilles Hertzog and me to take our usual places. As he pensively studies our photos of the army he commands, I imagine Mr. Poroshenko’s massive silhouette superimposed over the body of his smaller, younger successor.

Is Mr. Zelensky cut out for the role? Is it possible a former sitcom actor can be commander in chief? I watch as he recognizes in each picture the stretch of the front where it was taken, and sometimes the officer. I listen as he worries about the European Union being weakened by its leniency toward Mr. Putin, and he talks up the solidity of his ties with France and President Emmanuel Macron. I note his mastery of official-speak as he turns to the dark American drama that he unwittingly helped trigger through a phone call with President Trump, who was impeached in December and will be acquitted in February. 

I conclude that, all things considered, he is muddling through pretty well. Three times, perhaps to avoid a question, he repeats that he feels “normal” and that nothing could be more “normal” than answering questions about the present state of mind of the Jew from Kryvyi Rih who became a television star and then president of this land of pogroms and blood. 

Does he mean that he is now a normal world leader, or that he’s the same normal young man he was when we first met, before his surprising election in 2019? It doesn’t matter. Predominant in him now is a serene, sardonic self-assurance that I hadn’t expected to find. History could have chosen a worse champion to defend the colors and values of Europe against Mr. Putin’s “Eurasian” imperialism. 

For us heedless Westerners, this forgotten war in Ukraine—with its drip-feed of tragedies, and, on this side of the 300-mile front, these good people who continue to stand guard two hours before midnight—should lie heavy on our collective conscience.