The protection of submarine data cables is receiving increasing attention. From the Baltic, through the Suez Canal and the South China Sea, to the cables connecting remote islands in the Pacific, there is a growing concern about the vulnerability and perceived threats arrayed against the backbone of modern civilization that is found on the ocean floor.  Many assessments focus on the protection of submarine data cables during peacetime or in the context of operations falling short of the threshold of armed conflict while fewer focus on the protection provided by international humanitarian law (IHL) during an international armed conflict at sea.

During both WWI and WWII, the cutting of submarine telegraph cables constituted some of the first actions of belligerents. If telegraph cables proved to be of strategic value to the military and of critical importance to civilians, contemporary submarine data cables are proving to be so many times over.

Isolated submarine data cable outages have already shown its negative effect on the availability of insulin, baby food, financial services and online banking, cash, medicine, medical services and means to secure livelihoods, to name but a few. When one includes the increasing dependence of critical infrastructure like powerplants, water treatment plants, water supply installations, hospitals, municipal services, other healthcare services, and the sensitivity to cable cuts of the entire global financial system, global supply chains, family communications, and accessing documents and data in general, the possible large-scale targeting of submarine data cables during future armed conflict becomes a major humanitarian concern.

Although civilian owned submarine data cables are used primarily by civilians, the same cables are also used by militaries for command, control, communication, intelligence, surveillance, and conduct of military operations. Belligerents are to distinguish between military objectives and civilian objects, and are to direct attacks only against military objectives, otherwise known as the principle of distinction. Civilian objects are ordinarily protected from attack. However, civilian objectives that, by their military use, make an effective contribution to military action and whose total or partial destruction in the circumstances ruling at the time, offers a definite military advantage, may become military objectives. It is likely that belligerents in future large-scale armed conflicts at sea could regard such submarine data cables as military objectives and would consider the targeting thereof if a military advantage is to be gained.

Before a civilian object that has become a military objective is targeted, IHL requires a proportionality assessment. This proportionality assessment is best described in the Protocol Additional to the Geneva Conventions of 12 August 1949, and Relating to the Protection of Victims of International Armed Conflicts of 8 June 1977 (API). Article 51(5)(b) of API renders an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated as an indiscriminate attack that is prohibited. Article 57(2)(a)(iii) of API requires those who plan or decide upon an attack to take precautions that include refraining from deciding to launch an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage gained.

The large-scale targeting of submarine data cables will affect the civilian population, individual civilians or civilian objects on land. The result is that the API proportionality and precautions apply to the targeting of submarine data cables at sea during an international armed conflict in accordance with Articles 49(3) and 57(4) of API. Customary international law applicable to armed conflict at sea incorporates the same obligations meaning that it is irrelevant whether a State is a Party to API as it is bound to apply these principles notwithstanding.

For a proportionality assessment to be done, it is essential that the expected incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof is assessed. During military operations this would be done through intelligence assessments forming the basis of the information available to the decisionmaker at the time. It is unlikely that any military decisionmaker will be able to fully comprehend the extent of expected incidental damage, injury or death likely to result from large-scale cable cuts as the intelligence staff would find it very difficult to quantify the extent due to its extraordinarily far-ranging and complex nature.

What is certain is that the expected incidental damage, injury or death or large-scale cable cuts will be significant. The incidental damage, injury or death would highly likely be grossly excessive in relation to the concrete and direct military advantage gained if military operations of an adversary are impacted. Militaries have other means of maintaining their capabilities through satellite, radio, laser, and other dedicated means of transmission of data, whereas civilians mostly do not. Satellite based data transmission for civilians is estimated at being capable of handling only a small fraction of the capacity of undersea data cables and civilians are likely to suffer far worse consequences from large-scale cable cuts than an adversary would.

The development of laws protecting telegraph cables provides a useful reminder of the lengths that States may go to protect an element of critical civilian infrastructure. The Annex to the 1907 Hague Convention IV respecting the Laws and Customs of War on Land dictates that ‘[s]ubmarine cables connecting an occupied territory with a neutral territory shall not be seized or destroyed except in the case of absolute necessity.’ The incorporation of a higher standard of military necessity by the inclusion of ‘absolute’ when dealing with submarine cables is viewed as a useful indicator of the importance attached to the issue by States. These views may be even more important today, considering the increased utility and criticality of contemporary submarine data cables.

Over the last 150 years there have been repeated attempts by experts to draft non-binding military manuals that reflect what was regarded as the customary international law of the time of drafting. In 1880, one military manual stated that the destruction of telegraphs and landing-cables is forbidden, ‘unless it be demanded by military necessity’. In 1913, another manual stated that a ‘cable connecting a neutral territory with the territory of one of the belligerents may not, under any circumstances, be seized or destroyed in the waters under the power of a neutral territory.’ It went further indicating that ‘[o]n the high seas, this cable may not be seized or destroyed unless there exists an effective blockade and within the limits of that blockade, on consideration of the restoration of the cable in the shortest time possible. … Seizure or destruction may never take place except in case of absolute necessity.’ With respect to a problem that still vexes thinkers today because of the multiple-nationality private-ownership of submarine data cables, the 1913 manual explicitly provided that ‘no distinction is to be made between cables, according to whether they belong to the State or to individuals; nor is any regard to be paid to the nationality of their owners.’ The only classification criteria were whether the cable connected belligerents, belligerents and neutrals, occupied territory and neutrals, or neutrals with each other.  By 1994, a further manual adapted the rules around telegraph cable to submarine data cables and leaned toward protecting all cables that were not exclusively serving a belligerent.  In 2020, another manual by experts stated that ‘submarine communications cables, whether or not connecting occupied territory with neutral territory, may not be seized or destroyed even if they are serving one or more Belligerent States. Belligerent States must take care to avoid damage to such cables, unless they qualify as lawful targets.’ In 2025, another manual applied protections relating to telegraph cables without reservation to submarine data cables.

The routing and use of contemporary submarine data cables are of such a nature as that a single cable may connect many different countries and may pass through multiple territorial seas and archipelagic waters. Data sent through these cables may travel through many different cables to reach its destination, and the cut of one cable may have an impact on the routing of data in other areas. It appears that it not possible – even for the owners or operators – to reliably identify the users of the cables. While, during an armed conflict, the destruction of a submarine cable between opposing belligerents does not appear to be prohibited per se, the prohibition on destruction of telegraph cables connecting neutral States may come under strain. The latter is complicated because the contemporary cables are interdependent, interconnected, and reroutes data effortlessly, making it very difficult to conclusively establish that there is no reliance on cables between neutral States when belligerent States use the internet for military purposes.

While there may not be a lot of contemporary State practice following WWII (the last large-scale international armed conflict that saw military operations resulting in the cut of submarine telegraph cables), it is important for States to reflect on the outstanding characteristics of the legal framework that protected telegraph cables as it included a higher threshold for targeting of cables and did not make a distinction based on State or private ownership or the nationality of the owner of a cable. These principles may be even more important when it comes to protecting contemporary submarine data cables.

It may also be useful for States to consider whether applying the protections for submarine telegraph cables as-is to contemporary submarine data cables is sufficient given the significant expansion of their criticality and importance, and the magnified humanitarian consequences associated with the targeting of contemporary submarine data cables. Would the expanded importance not perhaps justify an expansion in protection?

Finally, it seems clear that more needs to be done to understand and assess the role that submarine data cables play in sustaining human survival and critical civilian infrastructure, as well as the humanitarian consequences of possible future large-scale belligerent cable interruptions during armed conflict. The identification of risks and vulnerabilities should be followed by the implementation of pro-active measures to increase resilience of critical civilian systems and equipment relying on submarine cables. That could include alternative data transmission options, contingency plans, the publication and dissemination of critical civilian risks, and the intentional separation of military and civilian use of submarine data cables.

The immense humanitarian cost to civilians that the large-scale attacking of submarine data cables promises – for both belligerent and neutral States – demands concomitant seriousness in action by States.