The phone connection from my living room in Berlin to the battlefield in eastern Ukraine couldn’t have been better. First Lieutenant Alexei Chaban of the Ukrainian 17th Tank Brigade came in loud and clear, the sound of gunfire in the distance. “There’s some shelling going on,” he said. “If the line cuts out, it’s a mortar attack.” Chaban spoke in the same matter-of-fact voice of his Facebook posts from the front.

Chaban, 50, has become an Internet sensation since the weekend, when he posted an open letter to the mother of a Russian tank commander whose life he had spared during a skirmish last week. When I told him Tuesday afternoon that his letter had already been shared 17,000 times and liked by 8,000 Facebook users, Chaban was taken aback. His mobile Internet connection is excruciatingly slow, he said, so he had had no idea how popular he had become. “I guess it’s a big number, but I’m no expert in these things,” he said. “I wasn’t trying to do anything special. I’m just an average guy.”

It’s exactly Chaban’s ordinariness that has made him a hero. A reserve tank officer from his student days at Dnipropetrovsk’s mining school, the father of four voluntarily enlisted in July as the pro-Russian insurgency in the neighboring Donetsk and Luhansk regions became increasingly violent. Chaban left his 250-acre farm behind for a month of training before being sent to the shaky cease-fire line.

Like thousands of other combatants, Chaban took his smartphone with him. While Ukrainian government ministers and volunteer commanders often seem to spend more time writing Facebook posts than doing their jobs, social media have also allowed rank-and-file soldiers to stay in touch with friends and families. Geo-tagged social-media posts by Russian soldiers over the summer put a lie to Kremlin denials of military involvement in Ukraine. In wars past, soldiers spent downtime writing letters home. Today they hope they get a strong enough signal to post on Facebook.

Chaban is no different, and his Facebook page, written in Russian, is full of snapshots from the field and commentary on equipment. (It turns out a Belarusian night-vision scope is better than an American one.) On Tuesday, Chaban reflected on how soldiers get used to danger and described how his comrades blew up a rebel truck filled with ammunition. “This is war. After it’s over, it will affect people for a long time,” he wrote.

A day earlier, Chaban recounted a skirmish near the village of Sanzharivka, north of the surrounded Ukrainian outpost of Debaltseve, where he found a wounded Ukrainian soldier who had been run over by an enemy tank. “I don’t know how to communicate these feelings,” he wrote. “I’m even afraid to communicate these feelings and what I’ve seen to the civilian world. The scene I encountered screamed with horror.”

Chaban wrote his famous letter on Saturday night. In it, he addresses the mother of a Russian officer who survived a hit to his tank, together with his gunner and driver, on Jan. 22. “When they got out of their disabled vehicle, we just had to push a button in our tank and all that would have been left of them would have been a memory of our sinful world,” Chaban wrote. “We didn’t kill them. We let them go.”

Chaban goes on to explain to the mother that Ukrainians face a host of problems — corruption, crime, poverty, unemployment — and chased former President Viktor Yanukovych from office in February to have a better life. Chaban assures her that Ukrainians aren’t fascists who eat babies or rape disabled pensioners, but ordinary, peace-loving people who love their country and children. “Tell your son that making a living by depriving other people of their lives is NOT good. May he return home and find other work,” Chaban wrote. “May he live peacefully and not take sins on his soul.”

Chaban appealed to Facebook users to pass the letter on to the mother of the officer, who had left his mobile phone, with a Russian SIM card, in his tank. Chaban posted a picture from the phone, showing a worn, middle-aged man sitting atop a tank in fatigues and a black Russian tanker helmet. Chaban also included three phone numbers and a street address, presumably of the officer’s mother, that he’d found on the forgotten phone.

Reporters in the central Russian city of Voronezh confirmed that the three phone numbers were local: two were out of service, and the woman who answered the third, a certain Marina, said she had seen Chaban’s Facebook post but didn’t recognize the man in the photo. When reporters went to the address in the post, they met a pensioner named Tatyana Golubyatnikova who denied knowing the man or having any relatives fighting in Ukraine. The Russian journalists speculated that the Chaban post could be a mistake — or even a fabrication.

I also tried calling the three numbers and duplicated my colleagues’ results. “I have no idea how my number appeared on the Internet,” Marina said. “All the people I know live here. I’ve never seen that man.” Since the weekend, Marina said she had received phone calls from Ukraine, Moldova, and even Germany and Spain. “Of course, Russians aren’t fighting in Ukraine,” she said. “I can’t even imagine how two brotherly nations could fight each other.” Marina declined to give her last name, saying she didn’t need the unwanted publicity.

When I reached Chaban, I mentioned the doubts cast on his story by the Voronezh journalists. Chaban said that it was possible the owners of the phone numbers had been warned by the Russian authorities. He said there were other numbers on the officer’s phone that he hadn’t posted on Facebook.

I was curious why Chaban hadn’t taken the Russian tank crew prisoner. He and his men were in the middle of a firefight, confined to their tank, Chaban explained. “We could have shot them or let them go. We couldn’t have taken them prisoner. It wasn’t realistic.” Life-and-death decisions had to be made in a matter of seconds. Chaban told me he’s religious, though his faith isn’t the reason why he spared his enemies’ lives.

“I can’t say that at that moment I was thinking about God,” he said. “But you can’t kill unarmed people.” Chaban allowed that he might have reacted differently if he had lost a comrade in his five months in the war zone. “I don’t regret it. Why should we have killed them? I don’t think those three will fight anymore.”

The enemy tank, a T64-BV, wasn’t badly damaged. Specialists who examined it determined from its serial number that it had been based in Crimea, which Russia annexed in March. After repairing it, Chaban and his crew made it their own.

Chaban said the pictures and text messages he found on the tank commander’s phone kept him up at night. Finally, he decided to work through his feelings in a letter to the unknown soldier’s mother.

Chaban agreed with me that modern technology, especially smartphones and social networks, have created unimaginable opportunities for soldiers to stay in touch with the outside world. He usually speaks to his wife Svetlana twice a day. “For families it’s easier,” he said, then paused. “Or maybe more difficult.”

I was sitting at my dining table in Berlin. Chaban was sitting out a bombardment somewhere north of Debaltseve. We spoke in our two separate realities, connected only in time and by a 6-hour-old Facebook friendship.

“I hope they’ll let me go home in March. I’m tired,” Chaban said. He needs to plant his crops if he expects to make any kind of living this year.