Professor Alexander Orakhelashvili
The UK Government has published its position on allowing the US Government to use UK’s military bases in the Iran conflict. It has provided “the agreement for the US to use UK bases in the collective self-defence of the region includes US defensive operations to degrade the missile sites and capabilities being used to attack ships in the Strait of Hormuz.” It appears from this that the role envisaged for the US involvement would be symmetrical with the earlier position enabling, as it were, the US were to step into UK’s shoes.
The position statement places the use of force in the context of “the collective self-defence of the region”. The decision on that matter has to be initially at least made by relevant Gulf States (which according to the UK letter to the UN was obtained by the UK from at least three Gulf States, S/2026/140, 9 March 2026). The UK would not be entitled to extend those earlier decisions unilaterally and thus request the US involvement on behalf of those or other Gulf States. It is yet another question how lawful, feasible or practical it would be for the relevant Gulf States to request collective self-defence against Iran from a State (namely the US) which is intensively engaged in an ongoing aggression against Iran itself.
Pursuant to the Definition of aggression under General Assembly resolution 3314, aggression includes “The action of a State in allowing its territory, which it has placed at the disposal of another State, to be used by that other State for perpetrating an act of aggression against a third State”. On that account and on strict legal terms, the UK is not committing an original act of aggression against Iran because the territory on which the Diego Garcia base is located is not under UK’s sovereignty. It is the territory of Mauritius, who has authorised the UK to exercise certain rights and jurisdiction there. There is no indication that any authorisation from Mauritius as a territorial State has been obtained for US military activities against Iran. The issue is also covered by the 2025 Chagos Treaty, Article 3(5), and in practice too, it is treated as one dependent on UK’s consent to the USA to be given in specific cases.
Against this background, the legality of UK’s permission given to the US turns on several factors. The first factor is whether and to what extent the use by US forces of Diego Garcia against Iran would correspond to the UK’s own parameters of the use of force against Iran, as defined earlier (and subject to military operations not becoming armed reprisals). A key factor here is the difference between self-defence (which is lawful under limited circumstances) and armed reprisal (which is a priori illegal). Here it should be noted that the latest UK’s position refers to targeting Iran’s capabilities that are “being used” in a certain manner, in contrast to the past tense used in the earlier statement. But the problem seems to be a wider one because “to degrade the missile sites and capabilities being used to attack ships in the Strait of Hormuz” does not necessarily or automatically trigger the UK’s right to self-defence. Here as above, the position taken and request for military assistance putatively or potentially provided by States of nationality of relevant maritime vessels would weigh in heavily to determine whether and how self-defence (including collective self-defence) could be invoked by the UK to justify the US use of force in this context. On balance, therefore, legality of the use of force depends on factors wider than the understanding reached between the UK and the US. It also seems that the intention to authorise the American use of the Diego Garcia base is likely to be a rather limited one, given the use of words “being used” in the latest UK statement.
As for the legality of Iran’s targeting of the Diego Garcia base, several factors are relevant here as well. As pointed out earlier, Diego Garcia is not part of the UK territory, and hence the UK’s conduct cannot be judged in terms of primary rules stated in resolution 3314. In other words, the UK’s decision to allow the US to use the base cannot per se amount to an original, fresh or wholesale aggression against Iran. Instead, secondary rules of State responsibility would determine whether the UK’s conduct would amount to an illegal use of force and aggression against Iran, through UK’s participation in American attacks. If the US aircraft taking part in the aggression against Iran is one that has taken off from Diego Garcia or another UK military base, then the UK would be responsible for the damage caused to Iran under the principle of plurality of responsible States. But that is not the same as whether and when Iran could lawfully use force to target Diego Garcia. Here as in other contexts, jus ad bellum and State responsibility ought to be kept separate. Given that any unlawful use of force against Iran would come from an American aircraft launched from the base, the base itself could be lawfully targeted only after orders to launch the concrete offensive operation has been given and while the US aircraft is prepared for launch or is being launched, i.e. while the UK would be participating in the American armed attack against Iran. For, the UK cannot be guilty of complicity in the US armed attack and be targeted through Iran’s self-defence any more than the US itself is guilty of its original aggression and can be lawfully targeted through Iran’s self-defence.
Before the above limited time period, Iran’s use of force targeting Diego Garcia would amount to a pre-emptive strike and its essence would become indistinguishable from the current US/Israeli use of force against Iran or US/UK use of force against Iraq in 2003. After that limited time period, Iran’s use of force would be an armed reprisal, also given that the targeting the relevant US aircraft would no longer require targeting the UK’s base. Barring any further escalation leading to UK’s potentially more intensive involvement in hostilities against Iran, the grounds on which Iran could lawfully attack Diego Garcia remain rather narrow at this moment.
It is rather likely that, on operational and logistical terms, Iran might find it difficult to target US planes exactly within the above limited timespan. But international law is not a mirror-image of strategic or tactical convenience. Jus ad bellum is a regulator of the use of armed force, not a planner of military success. To permit the expansive use of self-defence by factoring in prevention of certain threats or strategic or tactical (dis)advantages of the State which claims to be acting in self-defence is not any substantially different from equating nuclear proliferation risks or accumulation by a State of military power to an armed attack. Wars started on such premises have not done any good to anyone and if, as some international lawyers have suggested, international law is not a suicide pact, it should not be used as a recipe for disaster either.