The Legality of the UK Permitting the US to Use British Bases to Stop Iranian Attacks on Shipping in the Strait of Hormuz

Yesterday, the UK Government announced a significant change in its policy of allowing the United States to use British military bases during its war on Iran, solely to stop Iranian attacks on countries in the Gulf – I previously examined that issue here, and there was also an extensive discussion in the comments to that post. Now, the UK Government has expanded the consent it had given to the United States to also extend to operations against Iranian assets being used to attack shipping in the strait of Hormuz:

Ministers condemned Iran’s expansion of its targets to include international shipping. They agreed that Iran’s reckless strikes, including on Red Ensign vessels and those of our close allies and Gulf partners, risked pushing the region further into crisis and worsening the economic impact being felt in the UK and around the world.

They confirmed that 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.

They reaffirmed that the principles behind the UK’s approach to the conflict remain the same: the UK remains committed to defending our people, our interests and our allies, acting in accordance with international law and not getting drawn into the wider conflict.

That middle sentence is key here – I’m sure it has been heavily lawyered, as it were – and I’ll come back to some of the language below. The bottom line of this post is that, while I have previously defended the UK’s position on the limited American use of its bases, I am less confident that this Hormuz expansion is legally justified. At a minimum, it is legally riskier than the previous position that the UK has taken even if, politically, the harm that Iran is causing to its neighbours and the global economy by blocking most shipping through Hormuz means that the UK is unlikely to suffer a great political cost for taking this risk.

Let’s start with points that are, to my mind, completely legally uncontroversial:

(1) The US and Israel are waging a war of aggression against Iran, in breach of the UN Charter.

(2) All states have a duty not to participate in, or otherwise assist, this breach of the prohibition on the use of force in Article 2(4) of the Charter.

(3) Iran has the right to self-defence under Article 51 of the Charter, but that right is limited by the customary criteria of necessity and proportionality.

(4) Iran’s uses of force against countries in the Gulf clearly go beyond what is necessary and proportionate, for two reasons: first, because (with some possible exceptions, which I will not discuss here – see, e.g., reporting on missiles being fired against Iran from Bahrain) the Gulf countries have not permitted the US or Israel to use their territories to attack Iran, and second, because Iran is obviously attacking civilian infrastructure in these countries to inflict economic harm on them (and the world). This means that Iran’s uses of force against Gulf countries are not defensive in character, but constitute armed attacks. (This position is also affirmed by the recent adoption of UN Security Council resolution 2817 (2026) condeming Iran’s attacks and by the level of support that resolution had in the wider UN membership.)

(5) Therefore, the Gulf counties have the right to individual self-defence, to repel Iran’s ongoing armed attacks against them. 

(6) This right includes not only interceptive action to destroy Iranian missiles and drones in their airspace, but also covers possible forcible action on Iran’s own territory, such as missile or drone launching sites and production and command facilities. That Gulf countries have not yet themselves used force in this way, but have limited themselves to interceptive action only, is not because they don’t have the right to do so, but because they’ve made a political decision to try to avoid further escalation.

(7) Gulf countries also have the right of collective self-defence, in that they can request any state in the world to assist them in taking defensive action against Iran. This would include all of the types of military activities in point (6) above, i.e. not just the interception of incoming ordnance. 

(8) The right of collective self-defence is exercisable only upon a victim state’s request. There is some controversy, however, on whether there are any requirements of manner and form that such a request would have to fulfil, in particular, where it would need to be public. As thing stand, the majority view is likely that no requirements of form exist – so that a confidential written note or a phone call would suffice. Form is more a matter of proving that the request was actually made, if this becomes an issue. For a recent, extensive and rigorous discussion of these points, see James Green, Collective Self-Defence in International Law (CUP, 2024), chapter 6.

(9) It seems apparent on the facts that some Gulf states have made collective self-defence requests, but these are not public as such and we therefore don’t know whom they specifically addressed and whether they were in any way limited in scope.

(10) Therefore, the UK (and other states) have every right to defend Gulf states by all necessary and proportionate means, including all types of force in point (6) above, as well as by sharing intelligence or putting their bases or other facillities at the disposal of other states exercising collective self-defence.

Again, to my mind all of these points are entirely uncontroversial. What is controversial is the question whether the two states that started this war, the US and Israel (but especially the former) could exercise the right to collective self-defence of the countries in the Gulf, and whether, in turn, the UK could put its bases at the disposal of the US but solely for the limited purpose of stopping Iranian attacks on Gulf states. I have argued that this would, in principle, be legally possible, so long as, as a matter of fact, the UK ensures that the US use of its bases remains strictly limited to the defense of Gulf states.

By contrast, others have argued that this would not be legally possible, for two basic reasons – first, because the US could stop its war of aggression on Iran, which is what caused Iran to attack its neighbours, which means that any US response in putative collective self-defence fails the necessity test; and second, because any US actions against Iran can’t be separated from its overall campaign of aggression (see the discussion in the comments to my previous post, as well as this post by Adil Haque on Just Security). I don’t agree with these two arguments. The first denies Iran’s agency, its choice to attack its neighbours, and simply subsumes all of the choices that Iran has made to the initial aggression by the US. The second is, to my mind, really a question of fact, and is contingent on the evidence. I would certainly agree with the point that, by allowing its bases to be used, the UK exposes itself to a huge degree of legal risk of becoming complicit in the US aggression, risk that essentially all other countries in the world avoided. However, whether that risk materializes depends on how the bases are actually used.

Which brings me to the new Hormuz extension of the UK policy. To my mind, this extension poses distinct legal questions. It seems to draw an equivalence between Iran’s attacks on Gulf states and its attacks on shipping in Hormuz. But these are not the same, for several reasons. First, if an attack on a ship happened in the territorial waters of Oman, that could be said to be an armed attack on Oman – but the same is not true if a ship is struck in Iran’s own territorial waters or on the high seas. Second, it is legally very problematic to argue that attacks on a state’s merchant shipping are an armed attack on that state, in the same way as if a state’s military vessel was attacked. We’ve actually been here before, when the US and UK used force against the Houthis in Yemen due to their attacks on merchant shipping in the Red Sea (for more discussion, see here, here and here). Third, and this is a question of fact, from what I gather most if not all of the ships that were struck did not fly the flag of any Gulf country, but third states such as Thailand, Japan or the Marshall Islands (see here and here).

To be clear, I’m not saying here that Iran’s attacks on shipping in Hormuz are legal – they’re not, or at least most of them are not, whether as a matter of IHL (the law of naval warfare) or the general law of the sea (on the complexities of which see here). What I’m saying is that these attacks do not easily qualify as an ‘armed attack’ in the sense of Article 51 of the Charter, which give rise to the right to individual and collective self-defence. That is, if a Thai ship is struck by Iran in Hormuz, it is difficult to see how this is an armed attack on any Gulf country, except perhaps Oman if the ship was, at the moment it was attacked, in Omani rather than Iranian territorial waters.

Therefore, looking at the language of the UK Government statement above, it is a bit odd, to put it mildly, to have the Ministers confirm that the prior agreement somehow already included the Hormuz attacks, as if this is not legally a pretty substantial extension of the prior agreement. It is also interesting to note the references to ‘Red Ensign’ vessels – i.e. UK merchant shipping – but I’m not sure how many such attacks actually happened, and again it is legally highly problematic to equate attacks on merchant vessels to an attack on the state itself. Then there is the reference to ‘collective self-defence of the region’ – but, as I noted above, the only state in the region that can plausibly claim to be the victim of an armed attack regarding ships passing through Hormuz is likely Oman.

In short, the statement the UK Government released yesterday fails to answer some fundamental questions on why US attacks against Iranian military assets on Iranian soil would be lawful, and distinct from its overall war of aggression, simply on the basis that the Iranian assets were being used to attack shipping in Hormuz. Because that’s the key problem – if the US strikes cannot be justified by self-defence, then the UK cannot legally allow its bases to be used for that purpose. And to answer this question we would need to know, at a minimum, whether any state in the Gulf, especially Oman, made further collective self-defence requests to the US or the UK that specifically concern Iranian attacks on ships transiting Hormuz.