Talks between the US and Iran took place on Friday, aimed at avoiding a new round of military conflict. While further talks are planned, the obstacles to an agreement remain significant and a further US attack on Iran remains possible.
The US has built up a major force in the region, President Trump has repeatedly threatened to use force, and there have been calls for attacking Iranian targets in response to the unprecedented killing of protestors in January. Nevertheless, the question of the legality of any military action has been disturbingly absent from the European debate over possible US strikes –in striking contrast to the European stance on upholding the UN Charter in defence of Greenland and Ukraine.
European governments have rightly stepped up their criticism of Iran over its violent repression of recent protests, its nuclear programme, missiles and its destabilising actions in the Middle East. British prime minister Keir Starmer said he supported Trump’s goals with Iran, European governments must recognise that an American attack on Iran, under the current circumstance, would violate international law.
None of the purported reasons for striking Iran now provide a valid legal basis-indeed, it is striking that Trump has failed to articulate any clear and convincing reason why strikes would now be justified. At a time when the laws on the use of force are already under severe strain, it would be a grave mistake for European countries to take any action that could further undermine them.
The UN Charter in crisis
The UN Charter prohibits the threat or use of force by one state against another except in cases of self-defence or when authorised by the UN Security Council. This law, the cornerstone of the postwar international order, has hardly been scrupulously observed since 1945. Yet in the last few years, Russia’s full-scale invasion of Ukraine and Trump’s attacks and threats towards Iran, Venezuela, and Greenland have brought international respect for the Charter to a moment of crisis. A further US attack on Iran, following its bombing of Iranian nuclear sites last summer, would only add to the damage.
When Israel attacked Iranian nuclear, military, and energy sites in June 2025, it claimed a two-part legal justification for its action. As it spelled out in its letter to the Security Council (as required when states act under the claim of self-defence), Israel believed there was already an ongoing armed conflict between Iran and Israel, due to its support for the “persistent and unlawful attacks” of Iran’s proxies against Israel as well as its own clandestine actions.
This meant that Israel, under its argument, was not launching a new war by striking back. At the same time, Israel argued that its action was “a measure of last resort” to prevent Iran getting a nuclear weapon, which would threaten Isarel’s security and its very existence. For its part, the United States justified its participation in Israel’s war on Iran as an exercise of the right of collective self-defence—in other words, it was acting to defend its ally, Israel.
Legal arguments under scrutiny
These arguments met with widespread criticism from legal scholars. Critics argued that even if an armed conflict already existed, this did not relieve Israel of the need to justify its actions as necessary and proportionate to its self-defence. And they argued that self-defence against an imminent or ongoing armed attack – the standard established by Charter law – could not extend to a case where Iran might obtain a nuclear weapon unless it was known that it would immediately use that weapon to attack Israel.
Nevertheless, as other scholars argued, Israel’s action (and therefore US support) could be justified if Israel genuinely had reason to believe that Iran had a manifest commitment to attack Israel with a nuclear weapon once it obtained one, and that this was the last possible moment to act in order to prevent it.
That claim appeared highly implausible last summer, not least because Israel attacked Iran at a time when Tehran was in the middle of negotiations with Washington. A similar claim of self-defence would be all the more implausible as the basis for attacking Iran now, since the 12-day war clearly set back Iran’s nuclear programme.
Trump himself has repeatedly claimed his Operation Midnight Hammer “obliterated” Iran’s nuclear program, leaving little room to now argue there is an imminent threat of Iran obtaining nuclear weapons. It should go without saying that US frustration at Iran’s approach to negotiations would not provide a legal basis for an attack in the absence of a convincing claim of individual or collective self-defence.
Trump has also suggested that he might attack Iran because of its violent repression of recent protests, which human rights groups say left over 6,000 dead with 11,000 cases pending review. Western capitals are rightly horrified by the brutality that Iran’s regime displayed. But there are other ways they could react – including targeted sanctions and efforts to help document and publicise the regime’s actions. Military action now in response to the regime’s crackdown would be unjustified and unlikely to help the people of Iran.
The misuse of the responsibility to protect
Despite the development of the norm of an international “responsibility to protect” civilians against mass atrocities, only two countries (the UK and Denmark) have clearly argued that it can justify armed intervention without authorisation from the Security Council. In other cases where countries intervened for humanitarian purposes without the endorsement of the Security Council, as NATO did in Kosovo in 1999, they tended to present it as an exceptional act that did not set a legal precedent.
In any case, such intervention could only be justified when it prevented an imminent threat to civilian life. Since protests in Iran now appear to have subsided, a military attack directed against Iran’s crackdown would be punitive rather preventing an imminent massacre, as the responsibility to protect norm requires.
Even though European leaders may share Trump’s views about the nature of Iran’s regime, they should avoid encouraging military action or taking steps that directly involve them in a US attack on the country. The UK has already moved military assets the region – such as Typhoon fighters that have been deployed to Qatar - claiming these solely intended to defend British allies in the region that could come under Iranian retaliatory attacks if the US strikes.
Some European countries may however be tempted to provide the US and Israel with support such as intelligence sharing. Such a step would clearly implicate the Europeans in offensive operations, exposing their troops to Iranian strikes. Even rhetorical endorsement of US action would put European countries on the side of a violation of the UN Charter.
Leaving aside the reasons why a military strike on Iran would do more harm than good, support for an unlawful act of aggression would lend European backing to the erosion of fundamental laws that stand in urgent need of reinforcement.
