Starlink satellites operated by Elon Musk's SpaceX provide high-speed internet - something modern war relies heavily on.
For Ukraine, it is vital for communications on the battlefield and effective drone warfare.
Using Starlink, Ukrainian drones can stay connected to operators over longer distances than conventional radio networks and are less vulnerable to jamming.
But a sudden speed restriction for drones connected to Starlink, Elon Musk's SpaceX company, on Tuesday night meant some battlefield "missions" were disrupted, according to a top Ukrainian official.
Serhiy Beskrestnov also said officials had not been told about the 100mph (160km/h) speed limits set overnight on Tuesday.
Throttling drones connected to the network at this relatively low speed makes Ukrainian efforts to intercept jet-powered Russian drones all but useless, according to the Institute for the Study of War (ISW).
Starlink is an internet network that uses satellites in low Earth orbit to provide connectivity, particularly in remote areas and battlefields.
It currently relies on more than 10,000 active satellites, making Starlink the world's largest satellite constellation.
It is owned by Elon Musk's SpaceX, and is the company's biggest revenue driver.
While Starlink is for civilian use, Starshield is its military-grade service, designed for use by the Pentagon.
Starshield relies on the Starlink network, but has greater security like enhanced encryption so that sensitive information can be transmitted without interception.
The US government has allowed Ukraine to use Starlink and Starshield in its defence against Russia. Musk cut Moscow's access to the service in February, giving Kyiv an advantage in the conflict.
Starlink is "the backbone of military communications in Ukraine", the ISW's George Barros told the BBC, and is used across Kyiv's naval, land, and air forces.
The satellites are crucial in drone warfare, so that operators can stay connected with the weapon during strikes.
The technology effectively allows "drones, artillery and command posts to function as a single real-time network where the time between sensor-to-shooter is minimised," Barros, who is the ISW's Director of Innovation and Open Source Tradecraft, told the BBC.
The Starlink network is also crucial to protect Ukraine against incoming Russian attacks, especially jet-powered Shahed drones, Barros said.
Shaheds fly at speeds in excess of 198mph, meaning SpaceX's restriction makes Ukrainian efforts to chase them down all but impossible.
With no ready replacement for the Starlink network, any speed restrictions have major implications for Ukraine defences.
With more than 10,000 active satellites in Earth's low orbit, Starlink is able to ping high-speed internet back to almost every corner of the globe, using terminals on the ground that users can easily buy and set up themselves. As Musk said, they have two instructions: "point at sky, plug in".
This makes them ideal for use on the battlefield.
Tom Whittington, electronic warfare expert and associate fellow at RUSI, told BBC Radio 4's PM programme that the satellite system allows Ukraine to move large amounts of data - like video, text and voice calls - in a similar way that smartphones do but without the infrastructure that phones need.
The speed restrictions "effectively preclude" anything connected to the network going above 100mph, Whittington said, making drone navigation difficult within Ukraine and Russian-occupied territory inside Ukraine.
Serhiy Beskrestnov - Zelensky's adviser - says he believes the restrictions are not a coincidence and are related to the latest strains between Kyiv and Washington.
US President Donald Trump has criticised Ukraine's long-range strikes on Russian oil refineries and called for a new leader - criticisms which have been echoed by Elon Musk.
Ukraine's complaint about the restrictions has drawn attention to both Starlink, and Starshield - a military-grade version of the network developed for the Pentagon by SpaceX.
Musk says the two systems are separate, but in many cases they share the same underlying satellite technology.
And in a post on X, external, Musk suggested the US government could be responsible for the slowdown afflicting Ukrainian usage of the technology.
"Starshield systems are owned and controlled by the US government," he wrote. "Any limits on speed or usage are decided by the US military and ultimately POTUS.
"Starlink is a commercial service owned and controlled by SpaceX. Usage of Starlink is limited to civilian or defensive activities and cannot be used for offensive military action, as doing so would be inappropriate for a commercial company."
Russian President Vladimir Putin launched a full-scale invasion of Ukraine in 2022, and Moscow currently controls about 20% of Ukrainian territory.
