ZAPORIZHZHIA, Ukraine—When Elon Musk flipped the off switch on Russian forces’ Starlink internet connections in February, Ukraine’s military went on the offensive.

Russian commanders had lost access to live video of the battlefield and communications with troops. Ukrainian soldiers moved in on Russian positions with little threat from drones—normally an omnipresent danger.

Now, Ukrainian forces have notched their biggest domestic territorial gains in more than two years, dashing Russian President Vladimir Putin’s efforts to gain leverage in U.S.-mediated talks.

Ukrainian advances following Russia’s loss of Starlink—the satellite internet service of Musk’s SpaceX—underscore how vital the commercial satellite internet system has become in modern warfare—and the control that Musk himself is able to exercise over the conflict.

At the start of February, SpaceX went live with a system that only allowed Starlink systems on an approved “white list” to access the internet in Ukraine. Starlink systems owned by Ukrainian forces were approved, while Russian ones weren’t.

Since then, Ukraine says it has retaken roughly 150 square miles of territory in the southern Zaporizhzhia and Dnipropetrovsk regions, where Russian forces had previously been advancing rapidly. February was the first month since 2023 when Kyiv regained more territory in Ukraine than it lost, according to open-source analysts.

Across the whole of the front line, Russia still holds an advantage, including a 3-to-1 edge in troop numbers, according to Ukraine’s top general. Emil Kastehelmi, co-founder of Black Bird Group, a defense and intelligence analysis organization, noted that most of the territory Ukraine had regained was open fields and small villages.

But the Ukrainian advances have provided a timely boost to a nation that has been fighting off the Russian invasion for more than four years and survived a winter with widespread power and heating cuts.

Front-line soldiers say that, while there were other factors that enabled Kyiv’s counterattack, the Russian loss of Starlink was essential to their advances.

“It came at a critical moment,” said a soldier who goes by the call sign Konosh, from the Timur Special Forces Unit in Ukraine’s military intelligence directorate. “Without Starlink, they were basically pushed back to Cold War-era communications.”

Although Starlink isn’t active in Russia, Moscow’s forces had been obtaining terminals from middlemen who sent them from other countries. Over the past several years, Starlink became essential to both sides in the conflict, according to Ukrainian troops and independent analysts.

At command posts, Russian officers would watch live drone feeds and deploy resources based on what they saw. The system let them coordinate instantly and securely with drone pilots, who would then dispatch waves of attack drones at Ukrainian targets.

Once Starlink was cut off, however, Ukrainian troops noticed a dramatic drop in the number of drones coming after them.

“Before, if the enemy spotted our group, even a single soldier, they wouldn’t let him go. They would throw everything at him,” said Oleksiy Serdiuk, the commander of the Brotherhood unit of the Timur Special Forces Unit.

He recalled a time in February when the Russians spotted a group of Ukrainian soldiers in a house. Two drones soon struck it, but no others followed immediately, because commanders couldn’t quickly communicate the position to other drone pilots, giving the Ukrainian troops time to move to a new position.

“That gap between detecting the target and reacting has become critical for them,” he said.

Certain equipment that relied heavily on Starlink—like ground drones—also became far less useful, according to Ukrainian troops.

In addition, Russian commanders had gotten used to sending instructions to infantrymen about where to move. An officer in Ukraine’s Russian Volunteer Corps in the Timur Special Forces Unit, who goes by the call sign Sever, said captured Russian soldiers told them they were sometimes deployed with a Starlink device and “had to send video confirmation of their location to prove they hadn’t deserted.”

“With Starlink, they tightly controlled units,” Sever said. “That level of control is now gone. Without Starlink, those soldiers are isolated. They don’t know what’s happening outside the houses where they’re hiding.”

Though Ukraine announced the new “white list” system for Starlink before it took effect, troops in the Zaporizhzhia region said the Russians were completely unprepared when they lost access to it in February.

In the first week, Ukrainian officers said, the Russian commanders kept telling soldiers the connection would be restored soon. Troops repeatedly tried to reboot their terminals, which then helped Ukrainian forces detect their locations and hit them.

Without Starlink, Russia was forced to rely much more heavily on radio traffic—which, unlike communications over Starlink, Ukraine was able to intercept.

“We started hearing direct instructions in the radio traffic,” said a signals-intelligence specialist for the Timur unit. “Where units should move, which settlements they were going to enter, what routes to take…Sometimes we heard a day ahead.”

Ukraine’s counteroffensive in the Zaporizhzhia region had already begun when Russia lost Starlink, and Kyiv’s forces saw an opportunity to sow further chaos.

Ukraine sent small groups to attack Russian rear positions, hoping that the Russians would believe a much larger force had broken through.

“It gave us a real advantage—we were able to exploit weaknesses at a moment when their system of control was destabilized,” said one company commander in the Ukrainian army, who goes by the call sign Luna. “In intercepted communications, we could hear calls to abandon positions, because they believed our forces had already broken through.”

During the first weeks of Russia’s Starlink outage, Ukraine was able to push the Russians back from the outskirts of Zaporizhzhia, the regional capital, sparing the city from assaults from most artillery.

However, Moscow is now building workarounds. Ukrainian troops said Russian forces were running communication cables between positions, and using shorter-range wireless internet systems and Russian and Chinese satellite services.

Russian forces are also working to find Ukrainians who will help them register Starlink terminals on the white list. Ukraine’s security services have already arrested two Ukrainian citizens in connection with these efforts.

So far, none of the workarounds have proved as effective as Starlink, according to troops and analysts. Ukrainian troops estimated that since losing Starlink, the Russians had recovered around 60% over their previous level of coordination.

Michael Kofman, a senior fellow at the Carnegie Endowment for International Peace in Washington, D.C., said Ukrainian forces are now hunting the internet bridges Russia is using: “Russian forces try to adapt by employing mesh networks and modems, but Starlink was a cheap and effective solution,” he said.

In addition, Moscow’s forces are having an internal fight about which communications systems troops should use. The Kremlin has moved to ban troops from using Telegram—an independently owned messaging app popular within the armed forces—in favor of a state-owned system called Max. However, military commanders have been reluctant to switch, because they don’t want Russia’s Federal Security Service, which owns Max, reading their communications.

That conflict could present more problems ahead for the Russians as they work out their front-line communications.

“The army won’t grind to a halt just because Telegram gets cut off, but there will be problems,” said Serhiy Beskrestnov, an adviser to Ukraine’s defense minister. “Problems will arise in coordinating work between groups, coordinating operations at the lowest tactical levels—platoon, company, battalion.”