A battlefield death that spelled the end for Russia’s opposition

He took part in every rally he could against the Russian leader, enduring years of brutal torture in prison and alienating friends and family in Moscow through his uncompromising stance.

Other dissidents fled into exile. Most tried to build easier lives abroad as Putin’s government suppressed all forms of protest while the war unfolded. But Dadin concluded that the struggle against Putin was now best fought in Ukraine, on the battlefields.

“The only path left is the armed one," he told a military recruiter in Poland before traveling to the front lines to fight. “Anything else amounts to a deceitful and cynical defense of our own inaction."

The arc of Dadin’s life—from security guard in Moscow’s suburbs to pacifist protester and, finally, to the trenches of Ukraine—reflects the narrowing options available to Putin’s few remaining opponents in the face of growing repression, and their compatriots’ overwhelming apathy.

Three years into the war, Putin’s opposition is divided and diffuse. Its activists lead marches through European capitals, publishing anti-Putin tirades and feuding with each other on social media. Others lie low in Russia. A small number risk jail or death by setting fire to army-recruitment offices or sabotaging rail lines inside the country.

Dadin calculated that a bigger gesture would be required. By leaving for Ukraine, he tapped into a thread of self-sacrifice that runs through Russia’s history: from Old Believers who set themselves on fire to protest the tsarist state, to Alexei Navalny, the Putin foe who returned to Russia to be arrested and died in an Arctic prison colony last year.

Even being killed in a foxhole was preferable to remaining on the sidelines. Several months before first firing a weapon in Ukraine, he told the recruiter: “If some day the people living in Russia will live better, then I’m confident my actions bring this moment closer."

Dadin’s initiation into Russia’s opposition movement began in 2011. He quit his dead-end job as a security guard to take part in a wave of protests then sweeping the Russian capital, after a series of stage-managed elections had tightened Putin’s hold on the country.

The movement had adopted a white ribbon as a symbol of its fight for transparency and peaceful protest, and Dadin, an admirer of Indian independence movement leader Mahatma Gandhi, was a natural fit.

Each time he was detained at the protests, Dadin would interlock his fingers to avoid accidentally striking one of the police officers, friends say. One time he addressed reporters through the barred window of a riot van after a severe beating by the police. “Putin will answer for his crimes before the Russian nation," he said, his white T-shirt drenched in his own blood.

Dadin was meticulous to a fault. He neither smoked nor drank nor cursed, friends say. He camped outside police stations where protesters were held. When activists gathered for a restaurant dinner after hours on the street chanting, “We will not go anywhere!", Dadin denounced their hypocrisy and went home in a huff, said Olga Romanova, a friend and fellow activist now living abroad.

“He was so uncompromising in his principles that it was uncomfortable," said Romanova. “You felt self-conscious about your nice apartment, about the butter in your fridge, about the decent shoes on your feet."

In May 2012, the Moscow protests turned violent. Demonstrators pelted police with rocks and beer bottles, snatching their helmets and hanging them on trees as trophies. Riot police responded with baton charges and pepper spray.

But far from swelling the movement’s ranks, the crackdown presaged its collapse. Rattled by the protests, which he said were incited and financed by the West, Putin passed a series of repressive laws. Dozens of activists were jailed. Many abandoned the fight, settling into a tense but less exhausting coexistence with the state.

Dadin, who had just turned 30, only increased his activism. His stocky frame and receding hairline were visible at every protest, from gay-rights rallies to demonstrations against plans for building over city parks. He made ends meet by taking on odd jobs and crashing on friends’ couches.

In 2013, he traveled to Kyiv to join the revolution that would topple Ukraine’s pro-Moscow government. Dadin saw how police violence, which in Russia had cowed the protesters into submission, had served in Ukraine to instead grow the movement’s ranks.

Back in Moscow in early 2015, he was arrested at another rally and charged with repeatedly joining antigovernment protests under new legislation now known colloquially as “Dadin’s law."

Before a judge handed him a 2 ½-year sentence, Dadin quoted Gandhi and told reporters he would willingly go to prison if it helped his cause. “If I flee, as some do, then I’ll be a passive accomplice to this deepening fascism," he said.

By the time he walked free, the opposition had fractured and had little appetite for the kinds of sacrifices Dadin extolled. He too was damaged. Prison torture had left him with epilepsy and a permanent stammer. His marriage fell apart. He broke ties with his father, who backed Putin. With political rallies now outlawed, he mostly stood alone near the Kremlin holding up anti-Putin slogans, heckled by passersby.

When Putin launched his full-scale invasion of Ukraine in February 2022, Dadin decided he would have more impact in Ukraine than he would in a Russian prison as the state’s crackdown deepened. He traveled to Poland and began inquiring about service in the Ukrainian armed forces. In spring 2023 he met in Warsaw with Denis Sokolov, a Russian who was helping Ukraine recruit fighters for units made up of Russian citizens vetted by Kyiv’s military intelligence agency, HUR.

“I want to either die in battle, ending the turmoil of my guilt, or clear my conscience enough so I can once again live in harmony," he wrote to Irina Belacheu, an old friend from the opposition movement who now lives in Kyiv. “Anything other than direct action will destroy me even further from the inside, and I cannot go on living like this."

Since many of the Siberian Battalion’s fighters had families in Russia, their identities were protected. Most didn’t know even the first names of the men they fought alongside, current and former fighters said. Dadin became known by the call sign he chose: Gandhi.

He quickly earned a reputation for both stubbornness and bravery. He was liable to turn petty spats over food rations into conversations about life’s higher truths, irritating fellow servicemen, but he was also remarkably eager to cut his teeth in battle.

Siberian Battalion soldiers said that when commanders used a chat on the Signal app to solicit volunteers for dangerous missions, Dadin was always the first to respond with a “+"—military code for assent.

On a mission near Avdiivka in November 2023, his first-ever taste of combat, he spent 12 hours treating a wounded Ukrainian soldier in the gray zone long after his unit had received an order to withdraw.

“He stayed with him to alleviate the final moments of his life," said Alexey Makarov, a fellow fighter who was wounded in that operation. “Simply so he wouldn’t die alone."

Dadin was one of the few Siberian Battalion fighters who never hid his identity, and Russia began to take notice. Putin compared the several thousand Russians who were fighting for Ukraine to Soviet citizens who joined the Nazis during World War II, while an anchor on Russian state television asked in 2023: “Surely Dadin doesn’t expect to get away with this treason?"

In early 2024 he joined Ukraine’s biggest unit of fighting Russians, the Freedom of Russia Legion. He also became increasingly well-known among the remaining dissidents. Sokolov, the man who recruited him in Poland, said Dadin could inspire others to stand up against Putin.

“His life is a symbol of what we wanted to build" in Russia, he said. “And what we will build, if we don’t die in the process."

Dadin’s willingness to serve where the fighting was fiercest brought him in October last year to one of the hottest sections of the front line, in Ukraine’s northeast.

On the morning of Oct. 5, he and another infantryman, a 23-year-old with the call sign Sokar, received an order over the radio to assist in the evacuation of a severely wounded soldier from their unit.

After spending four days in a foxhole under withering artillery fire, Dadin jumped into action. “He was so energized by the idea of saving someone that we rushed off with no time to properly plan our route," Sokar said.

They ascended to the top of a steep bank and sprinted to a windbreak, chased by Russian drones. Then they buried themselves into the side of a small hill, targeted by unseen enemy troops positioned nearby.

A bullet tore into Dadin’s right buttock, and another severed an artery in his left leg. Sokar dragged Dadin behind a tree and gave him first aid, calling his unit commander over the radio. He kept talking to Dadin, who was losing blood fast.

“Every time I asked him something, I could tell he was slipping further and further away," Sokar said.

The team commander arrived with another soldier, but a grenade dropped from a Russian drone hit Sokar with shrapnel and badly wounded the other soldier. The commander told Sokar they would have to abandon the other two. “We can’t get them out," he said. They sprinted back to base.

Dadin’s body lay for two weeks in no man’s land, exposed to the elements. The Ukrainians initially kept his death secret, to prevent Russia from retrieving his body for propaganda purposes. Collecting his corpse was deemed too dangerous until there was sufficient cloud or rain to conceal a recovery mission.

Finally, on a windy morning at the end of October, Dadin’s fellow fighters gathered in Kyiv to lay his body to rest. Flanked by Ukrainian flags, with many shielding their faces behind balaclavas, the Russians saluted him in turn.

Write to Matthew Luxmoore at matthew.luxmoore@wsj.com