The United Kingdom’s Use of Force Against Iran: Walking a Legal Tightrope?

On March 1, 2026, British Prime Minister Sir Keir Starmer announced that the United Kingdom is taking defensive military action against Iran. The announcement comes on the heels of his earlier denial that the United Kingdom played any role in the strikes launched by the United States and Israel on Iran on Feb. 28. 2026. These very different statements are underpinned by what appears to be a nuanced legal argument. The purpose of this analysis is to unpack the British position with reference to the rules governing the use of force and the events that have unfolded since Starmer’s initial statement.

The U.S.-Israeli attack on Iran

On the morning of Feb. 28, 2026, the United States and Israel launched a coordinated military assault on Iran, codenamed Operation Epic Fury by the U.S. Department of Defense and Operation Roaring Lion by Israel. Using a combination of cruise missiles, drones and manned aircraft, the two States struck targets across Tehran, Isfahan, Qom, Karaj and Kermanshah. The Israeli Air Force reported that it had engaged over 500 military targets in western and central Iran, using over 200 fighter jets in the largest combat sortie in its history. Among the targets was the compound of Supreme Leader Ali Khamenei, whose death was later confirmed by Iranian state media, along with several other senior military and intelligence officials. The stated objectives of the U.S. and Israeli operations include destroying Iran’s missile and other military capabilities, preventing Iran from acquiring nuclear weapons, and toppling the Iranian government.

Some of the justifications offered by the United States and Israel for their use of force against Iran are rooted in the language of self-defense. The White House “War Powers Report” to Congress stated, “These strikes were undertaken to protect United States forces in the region, protect the United States homeland … and in collective self-defense of our regional allies, including Israel.”  Israel’s Defense Minister Israel Katz described the strikes as a “preemptive attack” intended to “remove threats to the State of Israel,” while Prime Minister Benjamin Netanyahu portrayed the operation as one designed to “remove the existential threat posed by the terrorist regime in Iran.” 

However, these arguments are insufficient to justify the use of force under international law on any reasonable reading of the rules. Article 2(4) of the United Nations Charter prohibits the use of force between States, subject to only two exceptions: authorization by the Security Council and the exercise of the right of self-defense under Article 51 “if an armed attack occurs.” In the absence of Security Council authorization, self-defense is the only possible legal basis. However, the preconditions for self-defense are not met, as expert commentators have noted (see here, here and here). There was no Iranian armed attack on the United States or Israel preceding the strikes. Even on the broadest understanding of anticipatory self-defense, there was no imminent attack by Iran against the United States or Israel that would have called for the use of force as a last resort. In any event, the operations against Iran appear to be designed to systematically degrade Iran’s military potential, including its nuclear infrastructure, and to bring about a change in its government. As President Donald Trump put it, the operation seeks to destroy Iran’s missiles, raze its missile industry to the ground and annihilate its navy. These objectives raise questions not only about the proportionality of any purported exercise of the right of self-defense, but also contradict its defensive nature.

The British position

The United Kingdom had previously refused a U.S. request to use British military bases for its military action against Iran. Under a longstanding agreement, the United States is required to seek permission before using U.K. military bases for its operations. According to British media reports, the decision to deny the U.S. request was taken on the basis of legal advice from the U.K. Attorney General, reflecting concerns that providing such support could render the U.K. complicit in an internationally wrongful act. In addition to flagging the risk of complicity under the law of State responsibility, it is likely that the Attorney General also advised the U.K. Government that allowing British territory to be used by the United States for perpetrating an act of aggression against Iran would itself amount to an act of aggression by the United Kingdom, in line with the United Nations General Assembly’s Definition of Aggression.

However, on March 1, Starmer announced that the UK granted permission to the United States to use British bases for the “specific and limited defensive purpose” of destroying Iranian missiles at source. What changed?

As the summary of the legal advice provided to the Government explains, the United Kingdom believes that Iran is conducting “indiscriminate attacks against countries in the region,” some of which have threatened U.K. service personnel. According to the summary, under these circumstances, international law permits the United Kingdom to use force in the exercise of the right of individual self-defense and “to use or support force” pursuant to the right of collective self-defense to assist regional allies who have requested British help.

The language of the summary and other official statements is nuanced. They speak of “indiscriminate strikes” by Iran against civilians targets and refer to “unlawful attacks” perpetrated by Iran against “countries not previously involved in the conflict”. In his statement on March 1, the Prime Minister took pains to underline that the United Kingdom was not involved in the initial strikes on Iran and that it would “not join offensive action now.” 

At the core of the U.K. Government’s legal position thus lies a distinction between offensive and defensive military action against Iran. Although key members of Government have done their best to avoid expressing a view on the legality of the U.S.-Israeli operation, in the light of the initial refusal to allow U.S. forces to operate from British bases, it is very safe to assume that London still considers the military action by the United States and Israel to be unlawful. This helps explain why the United Kingdom is not prepared to join “offensive action” against Tehran. However, at the same time the United Kingdom is relying on the right of individual and collective self-defense to counter what it describes as “unlawful attacks” by Iran. Though not stated in express terms, this might imply that Iran is entitled to use force in the exercise of the right of self-defense, but that it has exceeded the boundaries of that right.  

Lawful self-defense on the part of Iran? 

Given its scale, there is no doubt that the U.S.-Israeli action crossed the threshold of an “armed attack” and therefore triggered Iran’s right of self-defense in accordance with Article 51 of the United Nations Charter and customary international law. Indeed, Iran has said as much by formally invoking its inherent right of self-defense and declaring that it considers “all bases, facilities, and assets of hostile forces in the region” as legitimate military targets. 

Iran’s military response began within hours of the U.S.-Israeli attack on Feb. 28 and has continued in sustained waves. Its operations have focused heavily on U.S. military installations across the Gulf. News reports (here, here and here) confirm that Iran has targeted several bases in the region, including Al Udeid Air Base in Qatar and the U.S. Navy’s Fifth Fleet headquarters in Bahrain,. while some of its missiles have reached Haifa, Tel Aviv and surrounding areas in Israel.

The exercise of the right of self-defense is subject to well-established conditions. The defensive action must be necessary, meaning that the use of force must be the only feasible means of responding to the attack and non-forcible alternatives must be unavailable or insufficient. The force used must also be proportionate to the threat faced.

Iranian attacks directed against U.S. and Israeli military installations actively involved in, or reasonably believed to contribute to, ongoing operations against Iran would seem to meet these criteria without too much difficulty. Operation Epic Fury has been described by U.S. Central Command as “the largest regional concentration of American military firepower in a generation,” using “air, land, and sea” assets stationed across the theatre. It is rather unlikely that the United States would not make use of its extensive military facilities in the region to stage and deploy these assets. For example, Al Udeid air base in Qatar serves as the forward headquarters of U.S. Central Command and the Combined Air Operations Center (CAOC), the nerve center that coordinates all U.S. air operations across the Middle East. Some of the units deployed on Epic Fury, such as Task Force Scorpion Strike operating LUCAS drones, are based at undisclosed locations in the Middle East. On March 2, three U.S. F-15E Strike Eagles flying in support of Operation Epic Fury were shot down by friendly fire over Kuwait, which implies that Kuwait has, at the very least, permitted access to its airspace. 

The fact that these U.S. bases and military assets are located in third countries across the Gulf region does not render the Iranian attacks upon them an unlawful use of force directed against the territorial integrity and political independence of those third countries. As noted earlier, a State that allows its territory to be used by another State for perpetrating an act of aggression is thereby committing an act of aggression itself. Gulf countries that have placed their territory at the disposal of the United States for conducting an armed attack against Iran have therefore opened themselves up to measures of lawful self-defense by Iran directed against U.S. assets used in that ongoing armed attack, either on the basis that the use of force against U.S. assets located on their territory is necessary under the right of self-defense, and thus constitutes an exception to the prohibition to use of force in Article 2(4) of the Charter, or on the basis that lawful self-defense is a circumstance precluding the wrongfulness of the infringement of their sovereignty and territorial inviolability. Moreover, should the support that these Gulf States provide to the United States amount to an armed attack in its own right, this would grant Iran the right to engage in acts of self-defense against them directly.

Unlawful attacks by Iran?

However, Iranian operations have not been limited to military objectives. While some strikes may have caused civilian casualties inadvertently, other attacks were most likely deliberately aimed at commercial infrastructure and other civilian objects across the region. The nature of these attacks and the targets they pursued raise questions about whether Iran’s exercise of the right of self-defense remains within the bounds of necessity and proportionality.

A list of ostensibly civilian locations and targets struck by Iran published by U.S. Central Command before the United Kingdom’s new position includes: Dubai International Airport, Kuwait International Airport, Zayed International Airport in Abu Dhabi, Erbil International Airport in Iraq, the Fairmont Palm Hotel and Burj Al Arab Hotel in Dubai, the Crowne Plaza Hotel in Bahrain, the Port of Dubai, and residential areas in Qatar and Bahrain. The UAE Ministry of Defense reported that Iran had fired 165 ballistic missiles, two cruise missiles and 541 drones at the country, of which 21 drones hit civilian targets, killing three foreign workers. In Kuwait, a drone targeted the international airport, causing injuries and limited damage. In Qatar, residential areas were struck by debris from interceptions. In Oman, two drones hit a commercial port and injured a foreign worker. 

These attacks call into question whether Iran’s targeting is compliant with the law of armed conflict. However, its repeated and seemingly deliberate attacks on civilian infrastructure also raise the question as to whether Iran has exceeded the boundaries of lawful self-defense. This is because the use of force against civilian objects and infrastructure which do not support the ongoing armed attack by the United States does not, by definition, serve any defensive purpose and attacking such objects is neither necessary nor proportionate under the law governing the use of force. As the International Court of Justice put it in a comparable situation, such objects simply are not a “legitimate military target open to attack in the exercise of self-defense” (Oil Platforms, para. 51; see also Adil Haque’s essay in 2023 on civilian harm exceeding the right of self-defense).

Based on the foregoing, it is reasonable to conclude that Iran’s use of force has exceeded the boundaries of lawful self-defense. Since its attacks on civilian infrastructure cross the gravity threshold of an armed attack, this in turn gives rise to the right of self-defense by the affected Gulf countries against further Iranian attacks that similarly fail to meet the conditions for lawful self-defense.

Moreover, not all of the affected Gulf States may be supporting U.S. operations against Iran. Iranian strikes directed against such countries would not qualify as lawful self-defense, but may amount to armed attacks engaging those countries’ right of individual and collective self-defense.

Why the United Kingdom is in a tight spot

As noted in the summary of its legal position, the United Kingdom is acting “to use or support force” to assist regional allies who have requested U.K. support in exercise of the right of collective self-defense. The mention of action “supporting” the use of force presumably refers to the United Kingdom’s decision to allow the United States to use British bases for conducting defensive operations against Iran. In principle, permitting the use of one’s territory in such a manner constitutes a form of non-forcible support that falls within the scope of the right of self-defense. On this reading, the United Kingdom’s decision to permit the United States to use British bases for defensive strikes against Iranian missile sites is an exercise of the right of collective self-defense with the affected Gulf countries, effectively instrumentalizing U.S. military operations for these purposes.

In addition to the right of collective self-defense, the United Kingdom is also relying on the right of individual self-defense. As Prime Minister Starmer pointed out, “Iran hit a military base in Bahrain, narrowly missing British personnel.” On March 2, 2026, a drone struck RAF Akrotiri in Cyprus, a British Sovereign Base Area, hitting the runway and causing what Cypriot officials described as “limited damage.” U.K. Foreign Secretary Yvette Cooper later confirmed the strike. Subsequently, it was reported that two further drones heading toward RAF Akrotiri were intercepted. RAF Akrotiri is not merely a military installation on allied territory: it is a British Sovereign Base Area, forming part of the UK’s sovereign territory under the Treaty of Establishment of the Republic of Cyprus. An armed attack on RAF Akrotiri is therefore, as a matter of law, an armed attack on the United Kingdom that engages the United Kingdom’s inherent right of individual self-defense.

The logic of the United Kingdom’s legal position is clear: by distinguishing between “offensive” and “defensive” action against Iran, the United Kingdom is taking and supporting only military action that falls within the exercise of individual and collective self-defense against unlawful attacks by Iran, but not the wider U.S.-Israeli offensive campaign.​

Though the basic logic is legally compelling, the practical difficulty with this position is that the United Kingdom must ensure that any support it provides to the United States remains confined to the lawful exercise of self-defense against Iran’s unlawful attacks on regional allies and the United Kingdom itself, without spilling over into supporting the ongoing U.S.-Israeli armed attack on Iran. This distinction is legally essential, but operationally fraught.

First, the U.S. military does not neatly separate “offensive” and “defensive” operations into distinct institutional or logistical pipelines. The same bases, command structures, logistics and intelligence systems that support defensive operations simultaneously support the broader offensive campaign to degrade Iran’s military capabilities and bring about regime change. Second, the distinction between offensive and defensive action is difficult to maintain at the level of specific targets. A strike against an Iranian missile launcher that is actively being prepared to fire at a Gulf State is plainly defensive. But a strike against a missile production facility, a storage depot or an associated command and control node may serve both defensive and offensive purposes simultaneously. Seemingly aware of this difficulty, the summary of the legal advice suggests that the UK will facilitate action “against missile facilities in Iran which were involved in launching strikes at regional allies.” Third, there is a temporal dimension. As the situation evolves, the factual basis for defensive action may shift. If Iran were to cease its strikes on Gulf States and the UK, the necessity for defensive measures would diminish and any continued use of British bases for U.S. operations against Iran would lose its self-defense justification. The UK must therefore continually reassess whether the conditions for lawful self-defense persist, rather than treating its initial authorization as an open-ended licence. Fourth, in exercising the right of individual self-defense in response to the incidents at RAF Akrotiri, the UK must comply with the requirements of necessity and proportionality. Assuming for the sake of argument that the drone strikes did cross the threshold of an armed attack, it is open to debate whether extensive military operations against Iranian facilities would be necessary and proportionate in response. 

In short, the United Kingdom is, at best, walking a legal tightrope. At worst, it is attempting to draw an impossible distinction, as Adil Haque has argued earlier. Its position is doctrinally coherent in the abstract. In practice, however, maintaining a distinction between “offensive” and “defensive” operations would require a degree of control and situational awareness over U.S. military activity conducted from British bases that the UK may simply not possess. The risk is that, having opened the door to “limited defensive” support, the UK finds that door difficult to close.

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