Unit and collective self-defence

Chapter 1 flagged some particular problems for the use of force arising at sea. The first is the question of whether the exercise of self-defence by a warship is an expression of the right of national self-defence or the separate stii generis concept of unit self-defence. This has implications for whether it is lawful to target opposing units which do not actually pose a direct threat. The issue of unit self-defence is also relevant to the problem of collective self-defence and the extent to which warships can defend foreign warships in the vicinity and will have implications for the question of the right to escort own-flagged and foreign-flagged merchant vessels. A further question is when the right of self-defence at sea arises - is it when an attack occurs, or is imminent, or may be anticipated? It is hard to imagine any mariner wishing to be the victim of modem naval weapons. Preventing an attack before it reaches its target would seem profoundly preferable, but the problem of assessing when an attack is occurring is an age-old dilemma. The use of force without clear necessity could result in needless death and destruction as well as the unenviable position of being the aggressor. On the other hand, the failure to use force when needed could lead to one's own death and that of one’s own personnel, loss and damage to valuable military equipment and the weakening of one’s own state. This chapter will consider the issue of the imminence of an attack and how to determine it.

Unit self-defence v national self-defence?

Given that a warship ‘is an expression of the sovereignty of the State whose flag it flies’,1 there is some debate over whether the exercise of self-defence by a warship is an exercise of national self-defence or, instead, a sui generis


The "ARA Libertad” (Argentina v Ghana) ITLOS Case No 20, Provisional Measures, t5 December 2012, 20

Unit and collective self-defence 33 customary international law exercise of unit self-defence.[1] While the practical outcome of firing guns or missiles might look the same, the difference between when unit self-defence or national self-defence applies and how much force may be used is significant. The essence of unit self-defence is that a warship or task group that is defending itself as a unit may only use force against those targets which immediately threaten that unit, whether they be opposing vessels, aircraft or shore installations, as discussed subsequently. This is a matter for commanders on the scene. Were the legal basis national self-defence, however, this would involve an aimed attack against a nation, or a group of nations, where they defend themselves collectively against an aimed attack. What is necessary and proportional for a nation, or group of them, to defend themselves is then a broader question. Targets may include anything which it is necessary and proportional to use force against in order to defeat the aimed attack against the nation or nations. This could mean a broader set of targets beyond those immediately threatening a warship, task group or shore battery. This would, therefore, be a decision for governments rather than commanders on the scene responding to an immediate threat. This level of conflict could then cross the threshold into aimed conflict, in which case it would be lawfill to target any of the combat forces of the enemy, not just those which posed an immediate threat. The difference between unit and national self-defence is therefore significant in determining the degree of force that a warship or task group, or unit opposing them, may use in a navigational incident.

This means that there can be uses of force below the threshold of aimed attack within the meaning of article 51 of the UN Charter. Any attack on a warship would not necessarily be an attack on the flag state of that warship or military aircraft. Similarly, any response in order to repel the attack would not necessarily signify that an aimed conflict was now underwaybetween the states concerned. Further, obligations to report to the UN Security Council or enact mutual defence treaties would not necessarily be enlivened.[2] While there is no authoritative decision of an international judicial body directly addressing these issues, there is some guidance in the case law. The Corfu Channel Case strongly suggests this conclusion without explicitly stating it. and there are separate and dissenting opinions in the other cases which do directly address the question.

Judge Schwebel stated in the Nicaragua Case:

Article 51. as is well known, was not inserted for the purpose of defining the individual right of self-defence but of clarifying the position in regard to collective understandings for mutual self defence. . . . These understandings are concerned with defence against external aggression and it was natural for Article 51 to be related to defence against ‘attack’. Article 51 also has to be read in the light of the fact that it is part of Chapter VII. It is concerned with defence to grave breaches of the peace which are appropriately referred to as armed attack. It would be a misreading of the whole intention of Article 51 to interpret it by mere implication as forbidding forcible self-defence in resistance to an illegal use of force not constituting an ‘armed attack’. Thus, it would, in my view, be no breach of the Charter if Demnark or Sweden used armed force to prevent the illegal arrest of one of their fishing vessels on the liigli seas in the Baltic. The judgment in the Corfu Channel Case is entirely consistent with this view.

The main judgment in the Oil Platforms Case only stated that ‘The Court does not exclude the possibility that the mining of a single military vessel might be sufficient to bring into play the “inherent right of self defence’”. Judge Simma went further in his separate opinion:

I am less satisfied with the argumentation used in the Judgment by which the Court arrives at the - correct - conclusion that, since the Ir anian mine, gunboat or helicopter attacks on United States shipping did not amount to an ‘armed attack’ within the meaning of Article 51 of the Charter, the United States actions cannot be justified as recourse to self-defence under that provision. The text of paragraph 51 of the Judgment might create the impression that, if offensive military actions remain below the - considerably high - threshold of Article 51 of the Charter, the victim of such actions does not have the right to resort to - strictly proportionate - defensive measures equally of a military nature. What the present Judgment follows at this point are some of the less fortunate statements in the Court’s Nicaragua Judgment of 1986. In my view, the permissibility of strictly defensive military action taken against attacks of the type involving, for example, the Sea Isle City or the Samuel B. Roberts cannot be denied. What we see in such instances is an unlawful use of force ‘short of’ an armed attack (‘agression année’) within the meaning of Article 51, as indeed ‘the most grave fomi of the use of force’. Against such smaller-scale use of force, defensive action - by force also ‘short of’ Article 51 - is to be regarded as lawful. In other words, I would suggest a distinction between (hillscale) self-defence within the meaning of Article 51 against an ‘aimed attack’ within the meaning of the same Charter provision on the one hand and, on the other, the case of hostile action, for instance against individual ships, below the level of Article 51, justifying proportionate defensive measures on the part of the victim, equally short of the quality and quantity of action in self-defence expressly reserved in the United Nations Charter.

Judge Simina then effectively explains the difference between national and unit self-defence as follows:

To sum up my view on the use of force/self-defence aspects of the present case, there are two levels to be distinguished: there is, fust, the level of ‘aimed attacks’ in the substantial, massive sense of amounting to ‘une agression aimée’, to quote the French authentic text of Article 51. Against such aimed attacks, self-defence in its not infinite, but still considerable, variety would be justified. But we may encounter also a lower level of hostile military action, not reaching the threshold of an ‘aimed attack’ within the meaning of Article 51 of the United Nations Charter. Against such hostile acts, a State may of course defend itself, but only within a more limited range and quality of responses (the main difference being that the possibility of collective self-defence does not


Ibid. 331—332; see Green n 3, 59

arise, cf. Nicaragua) and bound to necessity, proportionality and immediacy in time in a particularly strict way. [emphasis added]

The idea of unit self-defence has notable currency in some of the manuals available in English. The 2009 Multinational Rules of Engagement Handbook of the International Institute for Humanitarian Law of San Remo, Italy, draws the distinction between unit and national self-defence as follows:

b. Unit self-defence. Unit commanders have the right to defend then-unit and other units from their nation in the face of an attack or imminent attack. For some nations, the concept of unit self-defence is both a right and an obligation; whereas for others the concept is only a right. Some nations pennit the right of unit self-defence to be limited by orders from higher authority. Unit self-defence may be extended to units and individuals from other nations when authorised by the applicable ROE.

c. Protection of Others. This refers to the right to defend specified persons (who are not part of the Force) against an attack or imminent attack. For some nations, the right of individual self-defence or unit self-defence may not include the right to use force to defend another nation’s citizens.

d. National self-defence. As recognised in Article 51 of the United Nations Charter, refers to the right of a nation to defend itself against aimed attack, and for most nations, the threat of imminent aimed attack. Decisions on whether or not national self-defence will be invoked are retained at the highest levels of governmental or executive authority.

The point concerning ‘Protection of Others’ is relevant to collective self-defence, which arises later.

Although it does not use the term ‘unit self-defence', the German Navy Commanders ’ Handbook: Legal Bases for Operations of 2005 states that German commanders have a right and obligation to protect their ships and


Ibid. 333; this view may appeal- to sit against the view expressed in the Guyana v Suriname Arbitration that lawfill countermeasures may not include force. Suriname in this case was not threatened with any use of force from Guyana. The issue was the presence of a drilling rig in waters disputed between the two nations, which posed no threat of the use of force, Arbitral Tribunal Constituted Pursuant to Article 287, and in Accordance with Annex VII, of the United Nations Convention on the Law of the Sea in the Matter of the Arbitration between Guyana v Suriname, 17 September 2007, 147-148; see Patricia Jimenez Kwast, ‘Maritime Law Enforcement and the Use of Force: Reflections on the Categorisation of Forcible Action at Sea in the Light of the Guyana Sunname Award’ (2008) 13(1) Journal of Conflict & Security Law 49, 58-59

Unit and collective self-defence 3 7 crews, in international waters and also in national waters where the coastal state cannot or does not act to protect the warship.[3] This is not a decision for the Federal Government and is distinguished from situations of armed attack upon Germany or those its shares collective self-defence arrangements with.

The US Commander’s Handbook on the Law of Nava! Operations of 2017 specifically refers to unit self-defence, stating that:

Unit commanders always retain the inherent right and obligation to exercise unit self-defense in response to a hostile act or demonstrated hostile intent. A hostile act is an attack or other use of force against the United States, U.S. forces, or other designated persons or property, including force used directly to preclude or impede the mission and/or duties of U.S. forces. Hostile intent is the imminent threat of the use of force against the United States, U.S. forces, or other designated persons or property. The determination of whether or not a threat is imminent will be based on an assessment of all facts and circumstances known to U.S. forces at the time, and may be made at any level.11

The Royal Australian Navy Manual of International Law of 1998 was very clear in its assertion of the right and its authority in customary international law deriving from the Caroline Principles. It helpfully clarifies the concepts as follows:

  • 7.5 The Commanding Officer of a warship has the right to act in unit self defence to protect both his vessel and any other vessel under his command. This right exists in customary international law and carries with it an absolute though narrow discretion. In order to invoke the right of unit self defence there must be an ‘instant and overwhelming necessity for self defence leaving no choice of means and no moment for deliberation'.
  • 7.12 The right of unit self defence is quite distinct from actions which may be tuidertaken in national self defence. In an attempt to highlightthis distinction, one view describes the right of unit self defence as an ‘On the Spot Reaction’ in which the incident is based upon the use of counter force. This does not mean that the government may not construe the incident as forming the basis for the use of force in national self defence bitt does mean that an individual commander under the right of unit self defence cannot initiate more sustained action unless there is some ongoing and acute threat.

There is sufficient support in the case law and the practice of states as evidenced in these manuals to argue for the existence of a separate and more limited concept of unit self-defence. It permits the use of force in self-defence below the threshold of self-defence in response to an armed attack upon a nation.[4] There is a further question, however, of the extent to which this right extends to the protection of units with different nationalities, and, still further, of the escort of foreign ships. This leads to the concept of collective self-defence at sea.

  • [1] See Dale Stephens, ‘Rules of Engagement and the Concept of Self Defense’ (1998) 45 Naval Law Review 126; Charles Trumbull, ‘The Basis for Unit Self Defense and Implications for the Use of F orce ’ (2012) 23 Duke Journal of Comparative and International Law 121; Dieter Fleck, ‘Rules of Engagement for Mantime Forces and the Limitation of the Use of Force Under the UN Charter’ (1988) 31 Gentian Yearbook of International Law 165,175-180; cf. Sir Michael Wood, Chair and Professor Noam Lubell, Rapporteur ‘Final Report on Aggression and the Use of Force’ (Use of Force Committee, International Law Association, Sydney Conference, 2018), 7; Yoram Dinstein, War, Aggression and Self-Defence (6th ed, Cambridge, 2017) 261-263; Tom Ruys, ‘The Meaning of “Force” and the Boundaries of the Jus Ad Bellum : Are “Minimal” Uses of Force Excluded from UN Charter Article 2(4)?’ 108(2) American Journal of International Law 159,181 is more equivocal 2 e.g. the time taken to mount counter invasions of the Falklands/ Malvinas in 1982 or Kuwait in 1991 was months, James Green, The International Court of Justice and Self-Defence in International Law (Hart, 2009) 102-103
  • [2] It may not be necessary to report unit self-defence to the Security Council, see Nicaragua Case 195, although the US did this in respect of the events giving rise to the Oil Platforms Case, 181 2 Nicaragua 348; see Dale Stephens, ‘The Impact of the 1982 Law of the Sea Convention on the Conduct of Peacetime Naval/ Military Operations’ (1999) 29 California Western International Law Journal 283, 297-302 in Donald Rothwell (ed), Law of the Sea (A Research Collection) (Elgar, 2013) 705 3 Oil Platforms Case 195
  • [3] 2 58-61 3 United States Navy, Commander is Handbook on the Law of Naval Operations NWP 1-14M (2017), para 4.4.13 4 Australian Book of Reference (ABR) 5179, given the likely authorship of the late Professor Ivan Shearer - Captain RANR (Royal Australian Naval Reserve) as well as Judge ad hoc of the International Tribunal for the Law of the Sea - it is worthy of attention
  • [4] The distinction between individual and unit self-defence in domestic law is a different but nonetheless important question which is beyond the scope of this book. 2 As discussed in the Introduction to this book in respect of escort operations in the Arabian Gulf
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