Is the UK's intervention in Iran war legal?

The day after the US and Israel launched their war on Iran, and the subsequent Iranian counterattack against US bases in the Gulf, Prime Minister Keir Starmer declared that the UK government would intervene in the conflict in two ways.

First, by intercepting Iranian drones and missiles to protect states not previously involved in the conflict.

And second, by allowing the US to use British bases for “specific and limited" defensive action against Iranian missile sites used to attack Gulf partners.

In his speech on 1 March outlining the government’s position, Starmer explained that the only way to stop the threat from Iranian missiles "is to destroy the missiles at source, in their storage depots or the launchers which are used to fire the missiles”.

He said that the UK would not join the US and Israel in their offensive strikes but would instead focus on “defensive actions”.

Starmer told the UK parliament: “We were not involved in the initial strikes on Iran, and we will not join offensive action now, but in the face of Iran’s barrage of missiles and drones, we will protect our people in the region, and support the collective self-defence of our allies, because that is our duty to the British people.

“It is the best way to eliminate the urgent threat, to prevent the situation spiralling further, and support a return to diplomacy. It is the best way to protect British interests, and British lives. That is what this government is doing.”

But the US-Israeli strikes have already been widely viewed as unlawful under the United Nations Charter. Surely, the UK’s decision raises questions about the legality of taking part in the conflict.

Middle East Eye has previously explained that the US-Israeli strikes manifestly violate the rules governing the use of force under international law, being neither acts of self-defence nor authorised by the UN Security Council.

So, is the UK acting illegally in intercepting Iranian missiles and allowing US base access? The answer is less straightforward. 

To get some clarity, MEE spoke to five leading international legal experts. But first, let’s unpack the UK’s legal arguments.

What is the UK government’s official legal position?

London has taken the unusual step of outlining its legal position to justify its intervention in the war on Iran.

The core of its argument is that its conduct is lawful, proportionate, and restricted to collective self-defence.

On 1 March, the UK prime minister’s office issued a legal summary which failed to address the legality of the initial US-Israeli strikes, but which did include an unequivocal condemnation of Iranian attacks on its Gulf neighbours. 

It described Iran’s actions as “reckless”, “indiscriminate”, and requiring a coordinated defensive response “to restore peace and security and prevent further escalation of the conflict”.

The summary said that the UK is acting in collective self-defence of Gulf states that have formally requested support. It also said that international law permits such defensive action where force is the only feasible means to halt an ongoing armed attack, and that such use of force must be necessary and proportionate.

Based on this, the UK stated it would take the following measures: 

• The UK will use its military assets in the region to intercept drones/missiles targeting countries that had not been initially involved in the conflict.  

• In response to a US request, the UK will facilitate specific and limited defensive action against missile facilities in Iran which were involved in launching strikes at the UK’s regional allies.

The UK emphasised that:

• It is not participating in the broader conflict between the US, Israel and Iran.

• Its role is strictly defensive and aimed at preventing escalation.

• It will notify the UN Security Council under Article 51 of the UN Charter (more on that below)

Does Iran have the right to self-defence?

To assess the legality of the UK’s entitlement to collective self-defence against Iranian attacks, it’s necessary to determine whether Iran’s action was lawful in the first place and, by extension, whether Gulf states had a right to self-defence.

Article 51 of the UN Charter allows states to use force in individual or collective self-defence after an armed attack, provided the action is reported to the UN Security Council and continues only until the council takes measures to restore peace.

The five experts interviewed by MEE agree that Iran had a right to self-defence under international law, following the unilateral and unprovoked use of force by the US and Israel. 

Janina Dill, a professor of global security at the University of Oxford’s Blavatnik School of Government, told MEE: “Iran has such a right against the US and Israel as well as any state that has allowed its territory to be used for the initial aggression.”

Iran’s response, however, must adhere to the principles of “necessity” and “proportionality”.

“Necessity” means that defensive force is required because no peaceful alternative can stop the attack. And “proportionality” means the force that is used should be limited in its scale, duration, and intensity to only repel the attack. 

“If Iran’s response no longer meets the criteria of necessity and proportionality, its use of force will cease to have a defensive character and will become an armed attack,” explains Marko Milanovic, a professor of public international law at the University of Reading School of Law.

And, he says, Iran’s strikes in the Gulf appear to have exceeded lawful self-defence and constituted unlawful armed attacks for two reasons. 

First, Gulf states have not attacked Iran. Nor have US attacks been launched from their territory. 

Second, Iran’s response was neither necessary nor proportionate. It struck US bases that played no role in attacks on Iran. It hit civilian areas far from legitimate military targets. It killed and wounded many civilians in breach of international humanitarian law.

“Iran’s attacks on Gulf states are not justified; they exceed the scope of permissible self-defence, and themselves constitute armed attacks on those states, which are now entitled to respond to them by using force,” Milanovic told MEE.

In fact, this is similar to Iran's official legal argument before the International Court of Justice in 1993, in its case claiming that US naval forces unlawfully destroyed Iranian offshore oil platforms in the Persian Gulf in 1987–1988 during the final phase of the Iran–Iraq War.

At the time, Iran argued that self-defence must be directed at the specific forces carrying out the armed attack. A state may lawfully respond against the invading military units threatening its territory. But it cannot extend that response to separate targets elsewhere that are not directly involved in the attack, according to Iran’s submission to the court.

Ben Saul, professor of international law at the University of Sydney Law School, offers an alternate view. He suggests that Iran does have a right to strike US bases in neighbouring countries as part of its self-defence.

“Iran’s right of self-defence against the US includes the right to strike US bases in Gulf countries, even if those bases were not themselves involved in the aggression against Iran,” he says. 

Saul told MEE that while Iran cannot violate the territory of neutral states by attacking US bases not involved in aggression, things become more complicated if such bases were violating their neutrality.

What if, for example, such US bases were used as transit hubs for US forces, or sharing intelligence signals  -  even if not directly launching attacks on Iran?

In such a case, Saul says, “the host state must take action, and if it does not, then Iran could have a right to attack”.

Do Gulf states have a right to self-defence against Iranian attacks?

Milanovic argues that, under international law, “there is no right to self-defence against force being used in self-defence”.

This means that Gulf states have a right to self-defence so long as the Iranian attacks fall under the category of an unlawful armed attack. 

“In principle, Iran’s decision to target its neighbours is a separate act from the US/Israeli attack on it,” he says. “It chose to do this. It could have done something else, like focusing its fire solely on Israel or the US. It nonetheless decided to fire on civilian infrastructure in Gulf states.”

Due to the unlawful manner of the Iranian response, Gulf states have a right to self-defence. They can do that individually, collectively as a bloc, or as a request for collective self-defence from a partner like the UK.

“Gulf states are entitled to invite other states, including the UK, to help them defend themselves - this is what’s called collective self-defence,” says Milanovic. 

Does the UK have a right to ‘collective self-defence?’

The UK has received a request from Gulf partners to help defend them in accordance with Article 51 of the UN Charter. 

The UK can therefore act in collective self-defence of Gulf partners facing unlawful Iranian attacks. This includes shooting down Iranian missiles and drones, and defending UK forces and assets in the region, says Milanovic.

Saul, on the other hand, says that “collective self-defence by the UK does not extend to the protection of US bases in Gulf countries - only the countries themselves”. This is based on his argument, outlined above, that Iran has a right to strike US bases in Gulf countries. 

‘Collective self-defence by the UK does not extend to the protection of US bases in Gulf countries, only the countries themselves’

- Ben Saul, University of Sydney Law School

But even if the UK takes part in collective self-defence, this must be, again, “necessary” and “proportionate”, according to Oona Hathaway, a professor at Yale Law School.

“This is all made more difficult by the fact that the self-defence is necessitated by the ongoing unlawful attack by the US and Israel,” she told MEE.

“Necessity is made more challenging to assess by the question of whether the unlawful threat to the Gulf states by Iran could be halted if the US and Israel halted their ongoing unlawful attacks against Iran.”

This analysis does not necessarily make the collective UK-Gulf self-defence unlawful, she argues.

But Hathaway says: “The concern is that, by assisting in the Gulf states' defence, the UK is enabling and prolonging the US and Israel’s unlawful attack on Iran.

“At a minimum, the UK is under an obligation to call on the US and Israel to halt their attacks. Here, the UK has fallen far short.” 
 

Can the UK lawfully let the US use its bases to hit Iranian missile sites?

Here, the scholars disagree. At its core is whether the defensive elements of the American use of force can be separated from its unlawful campaign.

According to Milanovic, the UK’s decision to permit the use of its bases by the US is potentially lawful if strictly limited to the defence of Gulf states.

“If, say Qatar, asks the US to help defend it, it has the right to do so,” he explained. “And the UK has the right to permit the US to use UK facilities to do so. What the UK mustn’t do is allow the US to use those facilities to further wage its aggressive campaign against Iran.”

Other experts, however, do not share the same view as Milanovic.

Adil Haque, a professor of law at Rutgers University, says allowing the US to use British bases for any attacks amounts to aiding US aggression and so would be unlawful.

Like Hathaway, Haque believes that the criterion of the necessity of self-defence is not met, as the US can stop its unlawful attack on Iran, and that in turn would stop Iran’s attacks on the Gulf.

“It follows that the US may not use force against Iran in collective self-defence of the Gulf States,” Haque says,  “and therefore the UK may not assist the US in using force against Iran.” 

'It is not possible for the UK to facilitate US strikes that are aimed at defending the Gulf states without also facilitating US strikes that are part of its unlawful campaign'

- Adil Haque, Rutgers University Professor

Second, Haque points out that the UK risks being involved in offensive action by the US, since the US is already carrying out strikes on Iran's missile facilities as part of its ongoing unlawful campaign against Iran.

“It is not possible for the UK to facilitate US strikes that are aimed at defending the Gulf states without also facilitating US strikes that are part of its unlawful campaign,” Haque says. “The strikes are one and the same.

“The UK would not be assisting the commission of lawful acts by an actor that is also, independently, committing unlawful acts. The UK would be assisting the commission of unlawful acts.”

Dill agrees.

“The US cannot act in collective self-defence with the victims of Iran's disproportionate self-defence because the US is the aggressor,” she told MEE.

“Since the US cannot act in collective self-defence with these states, the UK cannot assist it in collective self-defence.”