Combating sabotage of submarine cables and pipelines is first and foremost a matter of law enforcement. The problem is that the international law of the sea, as traditionally understood, does not provide coastal States with sufficient authority to effectively respond to sabotage activities beyond their territorial waters (for an in-depth discussion, see here). The European Commission and the High Representative for Foreign Affairs have recently proposed that the legal framework for intercepting and boarding vessels should be carefully assessed to enhance the security of submarine cables in accordance with the UN Convention on the Law of the Sea. At the same time, NATO doctrine is clear that hybrid operations against Allies could reach the level of armed attack and could lead the North Atlantic Council to invoke Article 5 of the North Atlantic Treaty. In reaction to the Nord Stream incident, then NATO Secretary General Jens Stoltenberg reportedly told journalists that an attack on critical underwater infrastructure could trigger NATO’s collective defence clause. This position seems to be gaining support among NATO Allies. In May 2024, the President of the German intelligence service BND said at conference that the extensive use of hybrid measures by Russia would increase the risk that NATO would eventually consider invoking Article 5.
NATO’s current Secretary General, Mark Rutte, emphasized that “while […] law enforcement roles are for national authorities, we are best served when there is complementarity between NATO’s efforts to detect threats and deter aggression, and national efforts to enforce the law”. Regardless of whether Article 5 of the North Atlantic Treaty applies to attacks on submarine cables and pipelines in international waters, it is not excluded that such attacks may fall within the scope of Article 51 of the UN Charter (for a discussion on this blog, see here and here, for a detailed analysis, see here). The application of the concept of armed attack in these cases raises a number of problems. First, it is necessary to clarify against which State the potential armed attack would occur. This is particularly problematic if the sabotage takes place in international waters. Second, it must be determined whether the act in question, by its scale and effects, constitutes an armed attack. If it is accepted that only “the most grave forms” of the use of force qualify as an armed attack, the question arises as to whether the secondary effects, and in particular the longer-term non-physical consequences of the loss of functionality of a cable or pipeline, can be taken into account. Third, it is debatable whether the right of self-defence would also cover enforcement action, including non-consensual boarding, search, and seizure of suspect vessels, where the action does not constitute a use of force within the meaning of Article 2(4) of the UN Charter.
The Victim State
If an act of sabotage amounting to an armed attack is committed in the internal waters, territorial sea, or archipelagic waters of a State, that State can invoke the right of self-defence to respond to the attack. The assessment is less clear when infrastructure is attacked in international waters, that is in ocean areas not subject to the sovereignty of any State, including the exclusive economic zone (EEZ), the continental shelf, and the high seas. In the case of an attack on oil or gas platforms located in the EEZ and on the continental shelf, the coastal State could be considered as the victim State, as it has the exclusive right to construct and to authorise and regulate the construction, operation and use of such platforms in its EEZ and on its continental shelf. And it has exclusive jurisdiction over these installations and structures.
When determining the victim State in cases of attacks on submarine cables and pipelines in international waters, it may not be practical to focus on ownership or jurisdiction. In general, the attack could be seen as directed against the State that owns and operates the cable or pipeline and has jurisdiction over it (arising from the freedom and entitlement to lay and operate submarine cables and pipelines in the EEZ, on the continental shelf, and on the bed of the high seas). However, in most cases such infrastructure is built, owned, and operated by private companies or State-owned entities, or by consortia of private or State-owned entities under the jurisdiction of one or more States. It would be far-fetched to consider the States of incorporation of these entities as victims of an armed attack. Take the Nord Stream incident. If incorporation were decisive, even States such as France, the Netherlands, and Switzerland could have claimed to be victims of the use of force. It is not clear whether the impact on these States would be of sufficient gravity to justify the activation of the right of self-defence on their part, even if the economic losses suffered by the owner and operator of the cable or pipeline were taken into account (see below). Prima facie, the consequences would appear to be far more serious for the States connected to the cable or pipeline and dependent on its service.
Rather than looking for a somewhat artificial link based on ownership or jurisdiction, it could be more instructive to ask which States are the actual or intended primary targets of the attack. Here, the international political and strategic context in which the sabotage takes place is of paramount importance. The situation becomes clearer the more obvious the nexus to an ongoing conflict and the more the sabotage fits into a pattern of hybrid warfare.
While it is relatively easy to identify which countries depend on the service provided by a particular pipeline or power cable, the complexity of Internet traffic and routing makes it more difficult to determine which countries are actually affected, and to what degree, when data cables are cut on the seabed. Nevertheless, the routes and landing points of submarine cables are commonly known and it can be assumed that the attacking State knows exactly which States are connected to the targeted cable. Therefore, in the absence of evidence to the contrary, an act of sabotage could arguably be presumed to be directed against the connected States (see also ILA Committee on Submarine Cables and Pipelines, p. 50). To the extent that cascading effects are taken into account in the assessment, it is not excluded that other States not directly connected to the attacked cable may also claim to be victims of the armed attack if they are seriously affected. But it is not settled to what extent the indirect and longer-term effects of the use of force are relevant in determining whether an armed attack has occurred, particularly when these effects are non-physical.
The Threshold
In Nicaragua, the International Court of Justice found that it is “necessary to distinguish the most grave forms of the use of force (those constituting an armed attack) from other less grave forms”. In describing what constitutes an armed attack, the Court focused on the scale and effects of the use of force. The United States, however, has long taken the position that the right of self-defence “potentially applies against any illegal use of force“ (Department of Defense Law of War Manual, para. 1.11.5.2). It appears that from the perspective of US doctrine and practice, damaging a cable or pipeline with hostile intent could in itself be sufficient to trigger the right of self-defence, regardless of any secondary effects.
Under the traditional reading of the scale and effects test, the physical damage to the cable or pipeline alone would probably not be sufficient to cross the threshold of an armed attack. It would therefore be necessary to show that the sabotage has effects sufficiently serious for Article 51 of the UN Charter to be activated. This issue has been widely discussed in the cyber context. The European Union and its Member States emphasized in November 2024 that “[t]he extent of any damage or destruction of property … or injury or death of persons, including as an indirect effect, are among the relevant factors for assessing whether the scale and effects of the cyber operation are grave enough to characterise it as an armed attack”. And Germany stressed that the strategic context and the effect of the cyber operation beyond cyberspace must also be assessed.
A cyberattack that disrupts air traffic control systems or the cooling process of a nuclear reactor could immediately cause massive destruction. In contrast, damage to a submarine data cable will not normally have a major physical impact in the countries connected to it. Nevertheless, the secondary and cascading effects of the loss of functionality of communications infrastructure could theoretically affect all aspects of life, including the performance of essential government functions, the maintenance of public order and security, the provision of public services, and all kinds of political, economic, and social activities. Highly interconnected States with sufficient backup and redundant cable infrastructure are unlikely to suffer extreme consequences if only one or two submarine cables are cut. But the situation is different for more vulnerable countries, such as small island States with only few undersea cable connections. From their perspective, any act of sabotage against this vital infrastructure could be perceived as an armed attack. Moreover, even highly interconnected States may become the target of a coordinated series of simultaneous acts of sabotage, strategically targeted at particularly critical parts of their infrastructure.
It is therefore necessary to clarify whether longer-term non-physical consequences that can reasonably be expected to result from coordinated sabotage campaigns could also be relevant in determining whether there has been an armed attack. In the literature, it has been argued that cutting a submarine cable will meet the gravity requirement of an armed attack “if it causes severe reduction in a state’s access to the Internet and other telecommunications for a substantial period of time”. However, the ILA Committee on Submarine Cables and Pipelines has emphasized that purely economic consequences or political disruption were not relevant in ascertaining the gravity of an attack. Ultimately, it depends on whether States are prepared to consider longer-term non-physical consequences as relevant in their assessment of a given situation. It is not unlikely that, in the face of the growing threat of strategically targeted sabotage, a broader understanding of what constitutes an armed attack on maritime cable infrastructure will gain traction among States.
Law Enforcement as Self-defence?
An open question is whether enforcement action against suspect vessels in international waters, where such action would not be covered by the international law of the sea, could be permitted as an exercise of the right of self-defence. This question arises because it may be argued that the right of self-defence can only be invoked to justify the use of force within the meaning of Article 2(4) of the UN Charter, and that maritime policing does not amount to the use of force. The more convenient view in the present context is that self-defence is a general right under international law, covering all measures necessary to repel an armed attack, whether they qualify as the use of force or are non-forcible in nature. This would include the interception and boarding of individual vessels. The latter argument is, of course, controversial and deserves further discussion.
Conclusion
Bringing the right of self-defence into play to respond to the most serious forms of sabotage of critical maritime infrastructure appears to carry a certain risk of escalation. However, the question is not whether the victim State may use military force against the attacking State. The more pressing question is whether the victim State may take enforcement action in self-defence against suspect vessels in maritime areas where the international law of the sea does not provide it with sufficient authority to do so. By adopting a complementary self-defence approach, States could increase their legal room for manoeuvre without overstretching the international law of the sea. And they could signal to potential attackers that they are prepared to act more robustly, if necessary, to protect their critical underwater infrastructure.