An issue with unit self-defence is the question of what is the unit? It seems reasonably clear from the preceding discussion that it would include a single warship or task group of warships of the same nationality. A problem potentially arises where the task group includes warships of more than one nationality, as is commonly the case. Can a warship use force to repel an attack on another warship in the same task group that is of a different nationality? This could include a situation in which a warship may use force where there is actually no direct attack upon it.
The Nicaragua Case accepts that there is a right of collective self-defence in customary international law but might appear to deny that warships could exercise unit self-defence in respect of foreign warships:
Thus the lawfulness of the use of force by a State in response to a wrongfill act of which it has not itself been the victim is not admitted when this wrongful act is not an armed attack. In the view of the Court, under international law in force today - whether customary' international law or that of the United Nations system - States do not have a
Unit and collective self-defence 39 right of ‘collective’ armed response to acts which do not constitute an ‘aimed attack’.
While an aimed attack would give rise to an entitlement to collective self-defence, a use of force of a lesser degree of gravity cannot, as the Court has already observed (paragraph 211 above), produce any entitlement to take collective countermeasures involving the use of force.
These statements were not made with the idea of multinational task groups of warships in mind. They would suggest that an attack on a task group of two warships of the same nationality could be seen as below the threshold of an aimed attack and therefore justify unit self-defence. On the other hand, if the two warships happened to be a task group of two different nationalities and used force in defence of each other, it would have to be an aimed attack to which the response would be collective national self-defence.
It is possible, however, to give effect to these propositions as put by the Court. Any acts of self-defence in a multinational task group should be authorised in advance by governments, most likely by way of an operational level agreement (not necessarily a formal treaty) between the governments and through rules of engagement from the government to its warships involved. Self-defence in this case would not be unit self-defence as such but rather collective national self-defence. This would be the case at least in customary international law and probably also within the terms of article 51 of the UN Charter, therefore require reporting to the Security Council should there be measures taken in self-defence. The important point is that warship or task group commanders could not decide themselves to act in defence of foreign warships: they must have the prior approval of governments. This view would be consistent with other statements of the Court:
There is no rule in customary international law permitting another State to exercise the right of collective self-defence on the basis of its own assessment of the situation. Where collective self-defence is invoked, it is to be expected that the State for whose benefit this right is used will have declared itself to be the victim of an aimed attack.
At all events, the Court finds that in customary international law .. . there is no rule permitting the exercise of collective self-defence in the absence of a request by the State which regards itself as the victim of an aimed attack. The Court concludes that the requirement of a request by the State which is the victim of the alleged attack is additional to the requirement that such a State should have declared itself to have been attacked.
Whilst expecting declarations by states that they are a victim right at the time of an attack on a multinational task group is unrealistic, it is reasonable that an agreement in advance between governments could authorise warships in a multinational task group to defend each other. The Royal Australian Navy Manual of International Law puts this view. This leads to the question of whether there is a right of collective self-defence in respect of civilian shipping.
Escort of foreign shipping
If it is possible to escort one’s own-flagged vessels in order to protect them, does this extend to escorting foreign-flagged vessels in order to protect them? The Oil Platforms Case made clear that this is at least possible:
Despite having thus referred to attacks on vessels and aircraft of other nationalities, the United States has not claimed to have been exercising collective self-defence on behalf of the neutral States engaged in shipping in the Persian Gulf; this would have required the existence of a request made to the United States ‘by the State which regards itself as the victim of an aimed attack’.
From this it is clear that escort of foreign shipping would require the request of the flag state. The request of the master or ship owner would be insufficient.
This raises an interesting question: does the obligation to render assistance to those in distress at sea under article 98 of the 1982 Convention includes using force to repel an attack, even where there is no prior
Unit and collective self-defence 41 connection between the warship rendering assistance and the vessel under attack? The request of the master of a vessel may be sufficient in situations where the threat came from non-state actors, such as pirates or terrorists, at least in international waters. Could it extend so far as repelling attacks from warships or military aircraft, or even shore batteries? Where the threat to the vessel was instead from sovereign acts of a state - whether law enforcement action or conduct in the course of international relations, such as to assert a maritime claim - then the consent of the master could not be sufficient authority to use force to protect the merchant ship. Being an act of self-defence against the sovereign act of a state, this could only be done at the request of the flag state of the vessel. The consequences of such an act for international relations between the coastal state, the flag state of the warship and the flag state of the merchant vessel would require decisions made by government, rather than at the tactical level.
-  Nicaragua 100 2 Ibid. 117 3 Ibid. 94
-  Ibid. 95 2 Paia 7.20 3 Oil Platforms Case 27; Natalie Klein is of the view that escort of neutral merchant vessels during an aimed conflict at sea, including those flagged to another neutral state, is accepted as a result of the Iran-Iraq Tanker Wai' in Maritime Security and the Law of the Sea (Oxford University Press, 2011) 290
-  On the distinction between tire two, see Kwast, n 8, 84-85; a use of force in international relations may be where there is an attack seeking to destroy a vessel or deter its presence in an area, whereas a law enforcement action would be more likely to be seekmg the apprehension or inspection of a vessel, see Guyana v Suriname Arbitration 147-148 2 Nicaragua 352 3 It is important to emphasise that anticipatory self-defence is not the same as the more controversial idea of pre-emptive self-defence, which concerns taking action against a threat before it becomes imminent, Ruys, n 2,175-176, and David Froman in ‘Uncharted Waters: Non-innocent Passage of Warships in the Territorial Sea’ (1984) 21 San Diego Law Review 625, 657, emphasise the importance of context in determining the legality of a forcefill response to an incursion; Yoram Dinstein, War, Aggression and Self-Defence (6th ed, Cambridge, 2017) 224 rejects ‘anticipatory’ self-defence but states that ‘self-defence can be invoked in response to an armed attack at an incipient stage’ 228 4 See Froman, ibid. 675