Early in their invasion of Ukraine, some Russian fighters closing in on the capital, Kyiv, made calls with cellphones and uploaded videos to TikTok, betraying their location to Ukrainian eavesdroppers.
The Ukrainians used the cellphone signals to launch missiles at their location -- to devastating effect, according to Ukraine's head of military intelligence.
Now, almost a year later and despite a ban on personal cellphones, Russian soldiers in the war zone are still using them to call wives, girlfriends, parents and each other, and still exposing themselves to Ukrainian attacks. After a strike that killed dozens -- possibly hundreds -- of Russian soldiers this week, one of the deadliest since the invasion began, the Russian military itself acknowledged the problem, using it to explain the heavy losses.
"It is already clear that the main reason of what took place included the massive use, contrary to the ban, of personal mobile phones in the range of enemy weapons," the Russian Defense Ministry said in a statement. The cellphone data allowed Ukraine, it said, to "determine the coordinates of the location of military service members to inflict a rocket strike."
Both a Ukrainian official and a group of Russian pro-war bloggers say other factors contributed to the strike and that the ministry was trying to deflect blame from military leaders by casting it on soldiers. Russian commanders had housed a large number of troops together rather than dispersing them, stationed them near munitions that detonated in the attack and failed to sufficiently disguise their movements, they said.
But the use of personal cellphones has plagued both Ukraine and especially Russia throughout the war, leaving troops vulnerable to a piece of technology that, however mundane and ubiquitous in daily life, can pose an existential threat in modern war.
Ukrainian officials say that Russian-backed forces have used cellphone data to target Ukrainian soldiers since at least 2014, when pro-Kremlin separatists began to fight Ukrainian troops in Ukraine's east.
The separatists debuted some of Moscow's newest forms of electronic warfare, Ukrainian officials say, and Ukrainian soldiers came to believe they were being targeted because soldiers -- often in groups -- were using their cellphones in proximity to one another. An artillery barrage on their position would soon follow the calls.
Almost a decade later, both Ukraine and Russia have honed their skills at using cellphone and radio signals as an effective targeting tool. While some Russian and Ukrainian units follow strict rules and ensure that cellphones are nowhere near front-line positions, social media posts from the battlefield show that cellphones are common among soldiers on both sides and that efforts to keep them away are uneven at best.
The extent of Ukraine's resulting losses are unclear, but they appear to be less severe than Russia's.
New York Times interviews with Russian soldiers and recorded phone calls intercepted by Ukrainian law enforcement throughout the war and obtained by the Times show that Russian commanders have tried, repeatedly, to keep phones off the battlefield.
Just before the invasion, Russian soldiers stationed in Belarus were told to give up their phones, two soldiers said in interviews. In intercepted calls, Russian soldiers can be heard saying that commanders confiscated their phones in February.
But just as often, soldiers found ways to circumvent the rules. They stole phones from Ukrainians, including those they had killed, and passed around the available phones to call home, an analysis of call logs shows. In many intercepted calls, Russian soldiers can be heard complaining that they did not trust their leaders or felt abandoned by them, and saying that they did not care about the rules.
Some Russian soldiers made remarks that showed they were aware Ukrainian intelligence could be listening -- and that they should choose their words carefully, to avoid giving away their locations. But the soldiers did not appear to know that cellphone data alone could potentially betray them, giving Ukrainians enough to pinpoint a phone's location down to an apartment building.
"Fighting against phones at the front in the 21st century is just as useless as fighting against prostitution, for example," a widely followed, pro-war Russian blog on the Telegram app said Wednesday. "It was, it is, and it will be."
The anonymous blogger said the use was not necessarily frivolous -- for example, Russian troops had used their phones to post messages on Telegram to direct artillery fire.
Some Russian generals spoke over unsecure phones and radios early in the war, according to current and former American military officials, enabling the Ukrainians to locate and kill at least one general and his staff through an intercepted call.
But the generals changed tactics after those strikes, analysts say, and high-ranking commanders appear to use safer communications than ordinary troops, an analysis of call logs shows. The commanders' phone numbers and those of their family members, for example, are conspicuously absent from call logs that the Times obtained from the Kyiv region in March, and Ukrainian officials say the commanders use an encrypted network.
Ukrainian soldiers believe the Russians look for Ukrainian cellphones "handshaking" with individual cellphone towers. Once either side establishes a pattern or locates the concentration of forces on their phones by other means such as drones, artillery strikes frequently follow.
In April in the eastern village of Husarivka, then just 3 miles from the front, a group of civilians found a spot in their tiny enclave where they could get cellphone service. But not long after a dozen or so residents congregated there to make calls, artillery shells started to rain down.
The pattern repeated itself to the point that almost everyone in the town kept their phones off or in airplane mode, and avoided gathering in any place for too long.
Despite the persistent threat, soldiers on both sides continue to hang on to their phones. The Ukrainians often have access to Starlink satellite internet near the front line, meaning calls do not use cell towers and are usually safe.
But even without Starlink, the pull to be connected to home and to family -- especially in such a brutal conflict, where even the homefront is targeted by Russian missile strikes -- is sometimes too powerful for Ukrainian troops to resist.
"What we didn't worry so much about 30 years ago now is every time you press a button, you're emitting," Gen. David H. Berger, commandant of the Marine Corps, said last month in remarks to the Defense Writers Group.
He said that commanders were acutely aware that young service members had grown up with cellphones and that their habits were deeply ingrained.
"They don't think anything about pressing a button," he said. "This is what they do all day long. Now we have to completely undo 18 years of communicating all day long and tell them that's bad. That will get you killed."
The weekend Makiivka strike seemed to be the latest blow to the Kremlin's military prestige as it struggles to advance the invasion of its neighbor.
However, said Emily Ferris, a research fellow on Russia and Eurasia at the Royal United Services Institute, "there should be a bit of caution around leaning too heavily on this [attack] as a sign of [the] Russian army's weakness."
As details of the strike have trickled out in recent days, some observers detected military sloppiness at the root of so many deaths.
U.K. intelligence officials said Wednesday that Moscow's "unprofessional" military practices were likely partly to blame for the high casualties.
"Given the extent of the damage, there is a realistic possibility that ammunition was being stored near to troop accommodation, which detonated during the strike, creating secondary explosions," the U.K. Defense Ministry said on Twitter.
In the same post, the ministry said the building struck by Ukrainian missiles was little more than 7.5 miles from the front line, within "one of the most contested areas of the conflict," in the partially Russian-occupied Donetsk region.
"The Russian military has a record of unsafe ammunition storage from well before the current war, but this incident highlights how unprofessional practices contribute to Russia's high casualty rate," the update added.
The Russian Defense Ministry, in a rare admission of losses, initially said the strike killed 63 troops. But as emergency crews searched the ruins, the death toll mounted. The regiment's deputy commander was among the dead.
That stirred renewed criticism inside Russia of the way the broader military campaign is being handled by the Ministry of Defense.
Vladlen Tatarsky, a well-known military blogger, accused Russian generals of "demonstrating their own stupidity and misunderstanding of what's going on [among] the troops, where everyone has cellphones."
"Moreover, in places where there's coverage, artillery fire is often adjusted by phone. There are simply no other ways," Tatarsky wrote in a Telegram post.
Others blamed the decision to station hundreds of troops in one place. "The cellphone story is not too convincing," military blogger Semyon Pegov wrote. "The only remedy is not to house personnel en masse in large buildings. Simply not to house 500 people in one place but spread them across 10 different locations."
Unconfirmed reports in Russian-language media said the victims were mobilized reservists from the region of Samara, in southwestern Russia.
The Institute for the Study of War saw in the incident further evidence that Moscow isn't properly utilizing the reservists it began calling up last September.
"Systemic failures in Russia's force generation apparatus continue to plague personnel capabilities to the detriment of Russian operational capacity in Ukraine," the think tank said in a report late Tuesday.
Ferris said the Makiivka strike shows the Russian army is more interested in growing its number of troops, not in training them in wartime skills.
"That's really how Russia conducts a lot of its warfare -- by overwhelming the enemy with volume, with people," she said. "The Kremlin view, unfortunately, is that soldiers' lives are expendable."
FRENCH TANKS FOR UKRAINE
Meanwhile, away from the battlefields, France said Wednesday it will send French-made AMX-10 RC light tanks to Ukraine -- the first tanks from a Western European country -- after an afternoon phone call between French President Emmanuel Macron and Ukrainian President Volodymyr Zelenskyy on Wednesday.
The French presidency didn't say how many tanks would be delivered and when. The NATO member has given Ukraine anti-tank and air defense missiles and rocket launchers.
In a grinding battle of attrition, Russian forces have pressed their offensive on Bakhmut in Donetsk despite heavy losses.
The Wagner Group, a private military contractor owned by Yevgeny Prigozhin, a millionaire businessman with close ties to Putin, has spearheaded the Bakhmut offensive.
U.S. intelligence officials have determined that convicts Wagner pulled from prisons accounted for 90% of Russian casualties in fighting for Bakhmut, according to a senior administration official who requested anonymity to discuss the finding.
The White House said last month that intelligence findings showed Wagner had some 50,000 personnel fighting in Ukraine, including 40,000 recruited convicts. The U.S. assesses that Wagner is spending about $100 million a month in the fight.
Information for this article was contributed by Alan Yuhas, Thomas Gibbons-Neff and Yousur Al-Hlou of The New York TImes and by Felipe Dana, Joanna Kozlowska and Aamer Madhani of The Associated Press.