US Marines join Baltic Sentry to protect undersea cables

Around 48 US Marines are being redirected to Finland to support NATO’s Baltic Sentry mission, using drones and littoral sensors to help monitor the undersea cables and pipelines that carry Europe’s communications and energy links. The move is a small reinforcement, but it shows how cable security has become a standing military concern rather than a niche maritime issue.¹²

The Marines come from 2nd Reconnaissance Battalion, 2nd Marine Division, and the II Marine Expeditionary Force Information Group. They will work alongside Finland’s Coastal Brigade and feed surveillance data into NATO’s shared operational picture. The US Marine Corps says the deployment is European-led, with its personnel providing a supportive, enabling role rather than taking command of Baltic Sentry.²

Baltic Sentry was launched in January 2025 after a series of incidents involving undersea infrastructure in the Baltic Sea. NATO’s activity combines naval patrols, aircraft, surveillance systems and other sensors to deter damage and improve the alliance’s view of what is happening below the waves.³

This is not an entirely new role for the Marines. Roughly 40 personnel from the same battalion deployed to Finland in February 2025, bringing RQ-20 Puma drones and coastal-operating experience. The new rotation builds on that relationship while adding another stream of reconnaissance data to a mission that depends on spotting suspicious activity early.¹²

The context is tense. Several cables and the Estlink 2 power connection were damaged in late 2024, while European governments have continued to examine the movements of vessels suspected of shadowing critical infrastructure. Those incidents do not, by themselves, prove who was responsible, and the latest deployment should not be presented as evidence of a new attack. It is better understood as a precautionary response to a vulnerable piece of infrastructure.¹³

The image accompanying this article is illustrative: it shows an underwater rope, not a Baltic Sentry operation or a subsea cable. Photo by Saad Alaiyadhi on Pexels.⁴

Our opinion

The useful detail here is the mission’s modest scale. A few dozen Marines and a handful of sensors cannot make hundreds of kilometres of seabed invulnerable. They can, however, make suspicious activity harder to ignore and give European allies a clearer shared picture. That quiet, evidence-gathering role is more credible than treating every deployment as a dramatic escalation, and it is likely to be the pattern for protecting undersea infrastructure while governments work out longer-term technical and legal safeguards.