As Keir Starmer considers whether Britain should support the US if Donald Trump decides to bomb Iran, the attorney general, Richard Hermer,has reportedly warnedhim that UK involvement could be illegal. The prime minister was an outspoken opponent of the Iraq war when he was a human rights lawyer in 2003 and will be well aware of the thorny legal issues around engagement in strikes against Iran.
British officials have repeatedly emphasised that the UK is not expected to deploy its military forces in any attack on Iran. Instead, a key issue would probably be whether to give permission for the US to fly B-2 stealth bombers from the Diego Garcia airbase in the Indian Ocean. Diego Garcia, which is the subject of a new 99-year lease agreement with Mauritius that leaves the UK in full operational control, is mainly used by the US. But the fact it is ultimately a British base means that Starmer would have to approve its use for an attack. RAF Akrotiri, Britain’s base in southern Cyprus, is also a potential launch site for US aircraft. Any use of the base by US forces would require the green light from the British government.
The UN’s founding charter outlines the principles governing the use of military force. There are three possible justifications: self-defence (which may include collective self-defence); exceptionally, to avert overwhelming humanitarian catastrophe; and authorisation by the security council acting under chapter VII of the charter. In the case of the bombing of Iran, the justification given by Israel – and one that would presumably be offered by the US and any of its allies – would be self-defence under article 51 of the charter. Force may be used in self-defence if there is an actual or imminent threat of an armed attack. It must be the only means of averting an attack and the force used must be proportionate.
The US has taken a broad view of “imminence” in cases of threats of terrorism or mass destruction in the past but it could prove difficult to argue that a US attack against Iran’s nuclear programme or leadership would constitute an act of self-defence against an imminent armed attack on the US. The White House would probably argue that it was acting in collective defence of Israel. The strength of this argument would rest on whether Israel has acted in accordance with international law in attacking Iran in the first place and then whether the US use of force was limited to protecting Israeli civilians and US interests from an Iranian attack.
Israel says its goal is to damage Iran’s nuclear programme and prevent it from developing a nuclear weapon. This is a broad interpretation of self-defence and its legality would depend on stressing the imminence of a nuclear attack. Israel would need to argue that it was the last window of opportunity to stop such an outcome. In 1981, Ronald Reagan’s administration backed a security council resolution that condemned Israel for launching an attack on the Osirak nuclear reactor in Baghdad. The resolution stated that “diplomatic means available to Israel had not been exhausted”. Comments from some Israeli politicians also throw doubt on the self-defence rationale.Israel Katz, the defence minister, said on Tuesdaythat the purpose of the campaign was “to remove threats to the state of Israel and undermine the ayatollahs’ regime”.
There is no distinction between a state carrying out the attack and those in support if the latter have “knowledge of the circumstances of the internationally wrongful act”, according to the UN. In 2021, John Healey, the defence secretary, asked in the Commons for clarification from the then Conservative government over the ground rules on the use of British bases by US forces. He was told that a proposed military operation would need to be accordance with UK law and the UK’s interpretation of relevant international law. The UK’s position on pre-emptive strikes is well known. In the lead-up to the Iraq war, the then attorney general, Lord Goldsmith, argued that international law permitted force only in self-defence where there was an actual or imminent attack and that “the development of [weapons of mass destruction] was not in itself sufficient to indicate such imminence”. Goldsmith argued only later that a UN resolution relating to Iraq made it a legal war.Writing in the Guardian at the time, Starmer, then a human rights lawyer, said article 51 might authorise a pre-emptive strike “in a nuclear world” but that any threat to the UK or its allies would have to be imminent and any force used in response to that threat would have to be proportionate. “The mere fact that Iraq has a capacity to attack at some unspecified time in the future is not enough,” Starmer wrote.