Warfare Across Domains


Traditionally, war was associated with a battlefield consisting of physical terrain across which opposing uniformed armies would face each other in the theatre of hostilities.1Rosa Brooks, ‘War Everywhere: Rights, National Security Law, And The Law Of Armed Conflict In The Age Of Terror’ (2004) 153 UPLR, p.710 Modern conflict now is no longer limited by terrain or even physical space with the advent of war in the air, at sea, in cyberspace, and in space. Much of this progression is due to technological advancement which has altered the means and the arena in which warfare is conducted. A ‘battlefield’ may now be multi-domain with attacks conducted in every sphere in which military capacity may be possessed. As it stands, states prefer to move this battlefield to domains of war in which they possess a military advantage, and also where their servicemen face fewer risk of casualties. These principles have guided much of the evolution of warfare and will be explored this month on the Diplomacy, Law and Policy Forum. The articles on this theme analyse the conduct of hostilities in the air, at sea, in cyberspace, and in space and all look at the historical development of this evolution, the ways in which they have been regulated, and the issues that arise from the rules of war applicable. These four domains will also be assessed briefly here.

Aerial Warfare

The advent of airpower as an emerging technology changed how war was fought and led to devastating consequences. This was first seen on a grand scale in World War II when aerial bombardment demolished cities such as Tokyo and Dresden causing much civilian suffering. It indicated the destructive power of the modern state and its ability to direct violence on targets beyond the front lines.2Kenneth Watkin, ‘Targeting in Air Warfare’ (2014) 44 Isr YB Hum Rts 1 Whilst its use is commonplace in armed conflict today, it was not always so. Hersch Lauterpacht in 1952 even said that: “Yet it is submitted, it may be unprofitable to inquire whether the practice of aerial bombardment in the form and to the extent which it assumed at the end of the Second World War was in accordance with international law …. The fact is that in the matter of aerial bombardment there is no rule firmly grounded in the past on which we can place reliance – for aerial bombardment is a new weapon which raises new problems.”3H. Lauterpacht, “The Problem of the Revision of the Law of War”, 29 B. Y.B.I.L. 360, 365-66 (1952) As a result, there were attempts to formulate rules applicable to aerial warfare.

The rise of aerial warfare started at the end of the nineteenth century when projectiles were lobbed from hot air balloons in 1783, however, they were too primitive to be taken seriously at that point in time. There were later attempts to regulate it in the twentieth century when aerial machines became more developed such as through the 1923 Hague Rules Of Air Warfare which were unfortunately not ratified by a single State. This was largely because States, as this technology developed, began to see its utility in conflict and did not want to stymie their ability to develop their air prowess. This development came to fruition in the Second World War. Strategic bombing campaigns destroyed cities and caused destruction of a kind not seen before in human history. For many the question was whether war from the skies could comply with the principle of distinction. This cardinal principle obligates States to distinguish between civilians and combatants and civilian objects and military objectives and to only target the latter.4Article 48, Additional Protocol I This was a particular criticism of the NATO operation in Kosovo in which targets were selected and fired upon from 15,000 feet in the air. Indeed, nowadays, aerial bombardment is often undertaken by States which are reluctant to send in their ground troops into hostilities. This reluctance often leads to the use of tactics which necessarily result in an acceptance of increased collateral damage. However, it is pretty much unthinkable now to conceive of outlawing air power given its widespread use by States over decades.

Lauterpacht’s words from 1952 sound so outdated to us now but they also ring true if we were to replace the words ‘aerial bombardment’ with ‘unmanned aerial warfare’. Much of the debate regarding the future of warfare focuses on the aerial plane and ‘unmanned’ or remotely piloted lethally armed drones.5George R. Lucas Jr., ‘Automated Warfare’ (2014) 25 Stan L & Pol’y Rev 317 This is because aerial warfare offers valuable opportunities for surveillance operations and can be used tactically and strategically.6ibid. Lethal autonomous weapons systems are able to operate without human supervision and select and attack targets themselves.7Paul Scharre, Army of None: Autonomous Weapons and the Future of War, 2019, W. W. Norton Company These lead to many attendant issues of reliability, accountability, vulnerability, and there have also been divisive debates surrounding the ethical and moral questions provoked by such weapons. There is not yet consensus on their lawful or otherwise use in warfare. It remains to be seen whether the international community will in fact regulate their use, or whether we will look back at this century as a time when neo-Luddites were trying to prevent the inevitable.

Hostilities at Sea

The modern history of the law of naval warfare can be traced back to 1856 when the maritime powers at the time signed the Paris Declaration which outlawed privateering.8Declaration Respecting Maritime Law (Apr. 16, 1856) This was the use of private vessels contracted by a State to attack enemy ships in war. Battle became the prerogative of State warships only. The Hague Convention (VII) Relating to the Conversion of Merchant Ships into Warships of 1907 also limited naval warfare to members of the armed forces and protected civilian-manned merchant ships from being targeted.9David T. Lee, ‘The Principle of Proportionality in Maritime Armed Conflict: A Comparative Analysis of the Law of Naval Warfare and Modern International Humanitarian Law’ (2021) 27 Sw J Int’l L 119 Civilian vessels were only subject to visit, search and capture.10ibid. However, the advent of the submarine and the British use of them to target German merchant vessels in World War I meant that these obligations were largely ignored.11ibid. These violations were repeated in World War II and enemy merchant ships and their civilian crews were regularly destroyed.12ibid. It was recognised then that the laws of war as they apply to land could not be readily applied to sea. This is codified in Article 49 of Additional Protocol I which states that Articles 48 through 67 “apply to any land, air or sea warfare which may affect the civilian population, individual civilians or civilian objects on land. They further apply to all attacks from the sea or from the air against objectives on land but do not otherwise affect the rules of international law applicable in armed conflict at sea or in the air.”

It was recognised that there was an inability to determine which laws then applied to naval conflict and so there was a need to codify and modernise these rules and a number of lawyers and naval experts convened in 1987 to discuss this very subject. Their work culminated in the San Remo Manual on International Law Applicable to Armed Conflicts at Sea which was launched in 1995.13San Remo Manual On International Law Applicable To Armed Conflicts At Sea (Louise Doswald-Beck ed., 1995) The law of naval warfare is lex specialis and is unique to the conduct of hostilities at sea with little parallel to the application of the laws of war on land.14David T. Lee, ‘The Principle of Proportionality in Maritime Armed Conflict: A Comparative Analysis of the Law of Naval Warfare and Modern International Humanitarian Law’ (2021) 27 Sw J Int’l L 119 This uniqueness can be seen in the fact that the cardinal principles of International Humanitarian Law (IHL) apply very differently in a maritime context. As Lee explains “conflicts at sea distinguish combatants from civilians by looking to the status or actions of the vessel alone to identify whether it constitutes a military objective…Status protects from harm select categories of vessels, including coastal fishing vessels, hospital ships, and merchant ships (enemy and neutral); they may be attacked only if they engage in certain specific conduct which reasonably can be construed as support to the enemy sufficient to warrant their designation as a military objective.”15ibid. Similarly, the principle of proportionality requires that states ensure that the collateral damage as a result of an attack is not excessive in relation to the concrete and direct military advantage sought.16Article 51(5)(b), Additional Protocol I However, individual protections as would be applicable on land to ensure that this principle is adhered to, do not exist at sea. A ship’s status as a military vessel or a civilian one determines targetability, without an individualised assessment of the civilian crew members which may be aboard.17David T. Lee, ‘The Principle of Proportionality in Maritime Armed Conflict: A Comparative Analysis of the Law of Naval Warfare and Modern International Humanitarian Law’ (2021) 27 Sw J Int’l L 119

Since the San Remo Manual there have been rapid technological developments which affect armed conflict at sea with some commentators subsequently arguing that there is a need to revise the applicable law to reflect current trends in technology.18Yusuke Saito, ‘Reviewing Law of Armed Conflict at Sea and Warfare in New Domains and New Measures: Submarine Cables, Merchant Missile Ships, and Unmanned Marine Systems’ (2019) 44 Tul Mar LJ 107 Similarly to aerial warfare, the future of military robotics will also occur underwater and in maritime zones in the form of anti-submarine attacks, surface combat, or for border security in the seas.19George R. Lucas Jr., ‘Automated Warfare’ (2014) 25 Stan L & Pol’y Rev 317 Single-manned vessels or unmanned underwater vehicles can operate to do what traditionally an entire crew would have done including, for instance, to search, find and report the presence of an adversary’s submarine.20ibid. Lucas argues that “[t]he development and increased use of unmanned systems in such environments could well lead to effective and relatively lowcost force multiplication with lower risk for combatants and noncombatants alike.”21ibid. As a result, much of the debate surrounding lethal autonomous weapons systems which have largely to date focused on killer aerial drones may shift underwater. The legal arguments surrounding their use will have to be applied to the sea, a very different terrain with different laws applicable.

Conflict in Cyberspace

The militarisation of cyberspace was an inevitability ever since the internet created a new digital domain in which hostilities could be conducted at a speed of milliseconds. Cyberoperations have become more of a real threat recently given the digitisation of much of daily life and the fact that technology now enables this domain to have kinetic effects. The World Cyber Warfare Statistics reports that cyberoperations have increased by 440% between 2008 and 2018.22DataProt, “The largest battlefield in history – 30 Cyber warfare statistics”, Nikolina Cvettićanin, June 8, 2022 The use of attacks in cyberspace along with other more traditional domains (such as land, air, and sea) are increasingly becoming the norm with States investing heavily in building their capacity to conduct cyber warfare.

Cyberspace, perhaps more than any other domain, has raised questions as to the continued relevance of the traditional rules of war when applied to this non-physical terrain. Indeed, the lacunas in the law when it comes to cyberattacks have led people to argue in favour of a Digital Geneva Convention which could better address the particular and unique issues posed by cyber warfare. A key challenge when it comes to the application of the rules of war to cyberspace is the question of how to define an attack in the cybersphere. This is a somewhat crucial question because the cardinal principles of IHL (namely, distinction, proportionality, and precaution) are all contingent on their being an ‘attack’. The Tallinn Manual on the International Law Applicable to Cyber Operations offers some help in this regard.23Tallinn Manual 2.0 On The International Law Applicable To Cyber Operations, Michael N. Schmitt ed., 2d ed. 2017 The Manual is a non-binding set of rules on the applicability of IHL to cyber military operations, and while not a source of law, is of persuasive value. It defines a cyberattack as a “cyber operation, whether offensive or defensive, that is reasonably expected to cause injury or death to persons or damage or destruction to objects”. However, this has been criticised as many cyberattacks may not cause injury or death but could instead lead to a ‘loss of functionality’ which would harm civilians. Some argue that the definition for the purposes of cyberspace only, should be broadened to include a ‘loss of functionality’. 

Another significant challenge with cyberspace is its use by non-state actors or those who want to remain anonymous to evade responsibility.24Brian M. Mazanec, ‘Constraining Norms for Cyber Warfare Are Unlikely’ (2016) 17 Geo J Int’l Aff 100 The challenges of cybercapabilities falling into the wrong hands are acute and have led to some arguing that there is a threat of a cyber Pearl Harbour or cyber 9/11.25ibid. This is especially since the destructive capacity of such operations has been demonstrated, for instance, through attacks such as Stuxnet on Iran’s nuclear reactors.26ibid. As per the International Law Commission’s Draft Articles for Responsibility of States for Internationally Wrongful Acts, 2001, conduct of a State organ, or agents under the control or instructions of the State, or conduct which is adopted by the State as its own conduct, is attributable to the State in question and thus the State is responsible.27International Law Commission, Draft Articles for Responsibility of States for Internationally Wrongful Acts, 2001 Articles 4 to 11 However, although there are many issues with the attribution of conduct in the physical and kinetic terrain, these are considerably greater in cyberspace. It is very difficult to attribute the conduct of hackers or disorganised entities who anonymise themselves to conduct attacks and may be sat anywhere in the world doing so. While not impossible, in that States may possess intelligence of a perpetrator’s identity or the location of the machine launching the attack, attribution in most situations of cyberoperations remains a Herculean task.

Battlefield in Space

States have sought to militarise space since the space war began in the 1950s when the first satellite was launched. Whilst now this is seen with actual plans to launch weapons in and from space, the Gulf War of 1991 was known as the ‘First Space War’ because the United States used its space capabilities in the form of satellites to guide munitions launched from land, sea and air.28Robert A. Ramey, ‘Armed Conflict on the Final Frontier: the Law of War in Space’ (2000) 48 AF L Rev 1 Since then, other States have also developed their space capacity having realised that this military advantage would be necessary in future conflicts. This has led particularly to the development of earth-to-space  attacks against orbiting satellites and the resultant countereffect of States then developing their defence space weapons in response to such attacks.29Kubo Mačák, ‘Military Space Operations’ ECIL Working Paper 2020/2, forthcoming in Sergey Sayapin (ed), An Introduction to International Conflict and Security Law (TMC Asser Press 2020)

The international law applicable to space is codified in five treaties, namely, the Outer Space Treaty (1967), the Agreement on the Return of Astronauts and the Return of Space Objects Launched into Outer Space (1968), the Convention on International Liability for Damage Caused by Space Objects (1972), the Convention of Registration of Objects Launched into Outer Space (1975), and the Agreement Governing the Activities of States on the Moon and Other Celestial Bodies (1979). The Outer Space Treaty is the most seminal of these as it has been signed by 109 states and, according to Maltz, it has  “formalized the demilitarization of the moon, banned the placement of weapons of mass destruction into orbit, called upon states to engage in only “peaceful purposes” in space, and encouraged consultations in case of planned activities that might harm another country’s space assets”.30James Clay Moltz, ‘The Past, Present, and Future of Space Security’ (2008) 14 Brown Journal of World Affairs 187 The regime as a whole has focused on peacefully exploring and using outer space cooperatively as is clear from the Outer Space Treaty’s preamble which recognises “the common interest of all mankind in the progress of the exploration and use of outer space for peaceful purposes”.311967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, 610 UNTS 205.

However, as already shown this has not prevented States from militarising this domain and using it for military purposes. IHL continues to apply in outer space to all activities which fall under an armed conflict though its application is not without issues. A key inconsistency between IHL and space law is in applying the principle of distinction. While IHL allows the targeting of combatants and military objectives, Mačák notes that under space law, all space crew are to be considered “envoys of mankind” and other states are to give them assistance in times of need.32Kubo Mačák, ‘Military Space Operations’ ECIL Working Paper 2020/2, forthcoming in Sergey Sayapin (ed), An Introduction to International Conflict and Security Law (TMC Asser Press 2020) He cogently poses the question of how this would apply to armed conflict in space, stating “how can a military astronaut of State A qualify as a lawful target and, at the same time, be entitled to all possible assistance from State B?”33ibid. His convincing proposal is that military astronauts maintain their status as an envoy of mankind under space law until they engage in hostilities under IHL.34ibid. The fact that the Pentagon has stated that it aims to test space-based weapons in 2023,35Patrick Tucker, ‘Pentagon Wants to Test A Space-Based Weapon in 2023’ Defense One (March 14, 2019) means that these questions may not remain hypothetical for much longer.


The terrain in which warfare is conducted continues to evolve in spheres not thought possible when the Geneva Conventions of 1949 were being negotiated nor the Additional Protocols of 1977. Technological developments and trends have legal, moral, and policy implications which the international community continues to grapple with. As States search for military advantage through advanced weaponry and across domains in order to have the best chance of success in warfare, the tactical uses and strategic consequences of this remain a cause for concern, not least for civilian protection. Aerial attacks are now seen as an inevitability of warfare, but this was not always the case. Similarly, as States increasingly start to use space as a domain in which hostilities may be more effectively coordinated, we see the same lack of international rules, allowing States to navigate this sphere themselves, gauging all the while how to benefit themselves militarily. Further regulation is required but the worry is that this may occur when such conflict is already an inevitability – resulting in effects for civilians which are seen as unavoidable. The world has changed so substantially in the last fifty years though that it is unlikely that laws negotiated when States still fought each other across physical battlefields will hold the fort for much longer. What these new regulations may look like, however, is anyone’s guess.