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Imminence is one of the more vexed issues in the international law of self-defence. It is arguably lawful to take anticipatory action against an imminent threat before receiving a potentially fatal or crippling attack. This is the ‘instant and overwhelming necessity for self defence leaving no choice of means and no moment for deliberation’ of the Caroline Principles. Note that in the Nicaragua Case, Judge Schwebel (in dissent) remarked that ‘it should be recalled that the narrow criteria of the Caroline case concerned anticipatory self-defence, not response to aimed attack or to actions tantamount to an aimed attack’. Imminence is therefore very closely connected to the Caroline Principles, as the principles concern the tactical and immediate more than the strategic and national. The difficulty is, when can one know that an attack is imminent?

A much-cited formulation[1] has found some official recognition in the Royal Australian Navy Manual of International Law. It suggests that the following actions may be indicative of an imminent attack at sea:

Objective indicia which have traditionally been regarded as relevant to a determination of intention to attack have developed over time. Thus the following factors (among others) should be considered:

an ‘opposing’ unit’s manoeuvring into a weapons launch profile;

the presence of a third party to coordinate over the horizon targeting;

the locking on of fire control radars, the opening of bomb bay doors;

the acoustic detection of toipedo or missile tube doors;

hostile electronic counter measures, and

the distance, speed and bearing of an opposing unit or force.

This is not to say that one of these factors alone should lead a commander to use force in self-defence; rather, some or all of these factors together with an assessment of ‘the prevailing political circumstances’ are ‘relevant to a determination of intention to attack’. This is plainly an imprecise process which will be clear in some cases, particularly where there has been a series of prior attacks, and not in others. It is where operational and legal considerations overlap closely.

The Multinational Rules of Engagement Handbook has developed these ideas further. It gives guidance on ‘hostile intent’, defined as ‘the threat of the imminent use of force’, which can be determined on the basis of capability and intention. (As mentioned previously, the demonstration of ‘hostile intent’ is also the basis in the US Commanders’ Handbook for the use of force in unit self-defence.) The Multinational Rules of Engagement Handbook states that there is no conclusive way to determine hostile intent but gives the folio whig examples:

a Aiming or directing weapons.

b Adopting an attack profile.

c Closing within weapon release range.

d Illuminating with radar or laser designators.

e Passing targeting information.

f Laying or preparing to lay naval mines.

g Failing to respond to the proactive measures to assist in determining the

intent of an opposing entity or force, including, but not limited to:

a Verbal query'.

b Verbal warning.

c Visual signals.

d Noise signals.

e Physical barriers.

f Changing course and speed to determine if continuing to maintain

an attack profile.

g Illuminating with fire control radar.

h Firing warning shots.19

These criteria are quite similar to those in the Royal Australian Navy Manual of International Law but are cast in more general terms and add the laying of mines and not responding to proactive measures. ‘Proactive measures’ is perhaps the most critical as interactive communication, or the lack of it, would appear to provide a strong indicator of the intent of a threatening ship or aircraft.

The most recent iteration of a document which seeks to address the issue of avoiding threatening situations at sea is the Code for Unplanned Encounters at Sea (CUES).[2] Rather than identifying criteria which may indicate hostile intent, it identifies actions to be avoided which may be misconstrued as an attack. Noting the previous point that the issue of imminence of attack is one where the legal and operational overlap, CUES is a document developed by the Western Pacific Naval Symposium (WPNS). This is a chief of navy-level professional forum. That is to say it is not a diplomatic forum or organisation, nor is CUES a legal document. CUES is a non-binding set of procedures for navies which was approved at the WPNS meeting of 22

April 2014 at Qingdao, China. Significantly, the Association of Southeast Asia Nations (ASEAN) Heads of State/ Government endorsed CUES on 7 September 2016, just after the decision of the South China Sea Arbitral Tribunal. Being a WPNS document, it does not apply to air forces, nor does it apply to coastguards, marine police, customs or other maritime law enforcement agencies.

CUES states its own purpose and character in its opening paragraphs:

  • 1.1.1 The Western Pacific Naval Symposium (WPNS) ‘Code for Unplanned Encounters at Sea’ (CUES) offers a means by which navies may develop mutually rewarding international cooperation and transparency and provide leadership and broad-based involvement in establishing international standards in relation to the use of the sea. The document is not legally binding; rather, it’s a coordinated means of communication to maximise safety at sea.
  • 1.1.2 CUES offers safety measures and a means to limit mutual interference, to limit uncertainty, and to facilitate communication when naval ships or naval aircraft encounter each other in an unplanned manner. Units making programmed contact should use procedures agreed between their national command authorities.

Paragraph 2.8 of CUES provides the key provisions as follows:

  • 2.8 Assurance Measures for Naval Ships
  • 2.8.1 Commanding Officers need to consider the potential ramifications before engaging in actions which could be misconstrued. Actions the prudent commander might generally avoid include:
    • (a) Simulation of attacks by aiming guns, missiles, fire control radars, torpedo tubes or other weapons in the direction of vessels or aircraft encountered.
    • (b) Except in cases of distress, the discharge of signal rockets, weapons or other objects in the direction of vessels or aircraft encountered.
  • 1

Joint Statement on the Application of the Code for Unplanned Encounters at Sea in the South China Sea, Heads of State/Govemment of ASEAN Member States and the People’s Republic of China, Vientiane, Laos, 7 September 2016; Cameron Moore, ‘The Arbitral Award in the Matter of the South China Sea Between the Philippines and China - The Use of Force and Freedom of Navigation’ (2017), Isia Pacific Journal of Ocean Law and Policy 117, 135-137; the South China Sea Code of Conduct negotiations apparently include consideration of de-escalatory measures as well, see Douglas Guilfoyle (ed), Strategy and Law in the South China Sea Disputes: Workshop Report (Maritime Security' Research Group, UNSW Canberra, 2019),13

  • (c) Illumination of the navigation bridges or air craft cockpits.
  • (d) The use of laser in such a maimer as to cause harm to personnel or damage to equipment onboard vessels or aircraft encountered.
  • (e) Aerobatics in the vicinity of ships encountered.

It is also worth noting that much of CUES is actually devoted to manoeu-vering and communications procedures. For the most part, the manoeuvering procedures reflect the Collision Regulations and the communication procedures are relatively simple. Whilst not being the same as the proactive measures of the Multinational Rules of Engagement Handbook, this does emphasise the importance of communication or the lack of it in determining the intent of a threatening unit.

As in the discussion of the previous manuals, most of the indicia in CUES cannot provide certainty that an attack is about to take place. Some indicia taken together with the operational context of the incident may provide a high degree of likelihood that an attack is imminent but, short of ramming or the firing of guns or missiles, there will not be certainty. By the time there is such certainty, it may be too late. The indicia in CUES have some similarities to those mentioned previously, with the notable difference being the use of illumination or lasers. Significantly, as discussed in the Introduction, there have been several reports of Chinese warships and other government vessels shining lasers into the bridges of warships and the cockpits of military aircraft. This underlines the nature of the problem in incidents involving freedom of navigation and the use of force. Provocative actions which could lead a commander to believe that an attack is imminent could be exactly the same as those intended to provoke an overreaction, where no


See the Commanders’ Handbook Part 2.10 and note the very similar provisions in the legally binding Agreement Between the Government of The United States of America and the Government of The Union of Soviet Socialist Republics on the Prevention of Incidents On and Over the High Seas (INCSEA) (United States and the Soviet Union) signed at Moscow 25 May 1972, entered into force 25 May 1972, supplemented by the.Igreement on the Prevention of Dangerous Military Activities (DMA) signed at Moscow on 12 June 1989 and the Joint Statement with Attached Uniform Interpretation of Rules of International Law Governing Innocent Passage (in Eric Francx, ‘Innocent Passage of Warships: Recent Developments in US-Soviet Relations ’ (November 1990) Marine Policy 484,489-490,; NB the US-China Military Maritime Consultative Agreement of 19 January 1998 (Commanders 'Handbook Part 2.10.3) has more limited provisions on similar subj ect matter. There are also Guidelines for Navigation and Overflight in the Exclusive Economic Zone developed by the Open Pohcy Research Foundation of Japan, see James Kraska, Maritime Power and the Law of the Sea (Oxford University Press, 2011) 230, Kraska also critiques the idea of a binding legal agreement between China and the US on such issues; see also Sam Bateman, Freedoms of Navigation in the Asia-Pacific Region: Strategic, Political and Legal Factors (Routledge, 2020) 80-81

attack is actually imminent but there is an intention to make an opponent appear to be the aggressor thr ough an overreaction.


The problem here is that the most difficult question of when to use force is as much an operational as a legal question. A legal analysis can only go so far in stating when it is lawfill to use force in an incident involving a warship. There can be no conclusive certainty in all cases as to when an attack is imminent, even if there might be certainty as to when an attack has reached its target. The answer to the question must be therefore highly contextual and subjective, assessed as much after the event as during it. The possibility of mistakes and miscalculations is highly significant and is an important aspect of the concept of unit self-defence. The fact that warships may end up in a fight does not mean that nations have to.

Unit self-defence is a sui generis right in international law that does not extend beyond that which is required for the immediate defence of the warship, task group or vessels which they are escorting, and may occur only against targets which pose an immediate threat. Escort of foreign-flagged vessels may only occur with the consent of the flag state. There can be collective self-defence arrangements in place to protect foreign warships or civilian vessels but these must be agreed between governments in advance, rather than by commanders on the scene at the time.

While it may be better to ensure that warships and military aircraft do not find themselves in situations where they have to defend themselves to begin with, this is not always possible. The question then is, if force must be used, then how much and for how long? This is the subject of Chapter 3.

  • [1] See Stephens, n 2, citing Fleck, n 2,181-182 2 Paia 7.11 3 Ibid. 4 See Klein, n 20, 266-267
  • [2] 22 2 In respect of the earlier edition of CUES in 2000 see Chris Rahman, ‘Naval Cooperation and Coalition Building in Southeast Asia and the Southwest Pacific: Status and Prospects’ (2001) Royal Australian Navy Sea Power Centre and Centre for Maritime Policy Paper No 7, at 31, who endorses the idea of greater cooperation through CUES, 57 and 62 3 Discussed in ‘Multilateral Naval Code of Conduct Aims to Prevent Unintended Conflict in Contested Areas of East and South China Sea’ in Kristina Daugndas and Julian Davis Mortenson (eds), ‘Contemporary Practice of the Umted States Relating to International Law’ (2014) 108(3) American Journal of Intel-national Law 529-532, 531; also Ronald O’Rourke, ‘Maritime Territorial and Exclusive Economic Zone (EEZ) Disputes Involving China: Issues for Congress’, Congressional Research Service Report, 6 June 2014,9
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