Israeli Air Force fighter jets head for Iran, June 2025. IDF Spokesperson's Unit / CC BY-SA 3.0

With no UN mandate and little disclosed intelligence, US and Israeli airstrikes on Iran have sparked global concern not just over war, but over the very rules meant to prevent it.

In June, long-simmering tensions erupted into open warfare when Israel launched ‘Operation Rising Lion’, targeting Iran’s nuclear facilities, military bases and senior officials. Israel justified the strikes as a pre-emptive measure based on intelligence. Tehran retaliated immediately, unleashing missiles and drones at Israeli cities and strategic sites.

‘Last night, history changed,’ Danny Danon, Permanent Representative of Israel to the United Nations, told the UN Security Council after the US strikes. ‘The United States, the leader of the free world, removed the greatest existential threat facing the free world...Just like our Operation Rising Lion, this was not a war of choice; this action was a necessity, it was a righteous act.’

Legally, the US and Israel defended their dramatic strikes by invoking the right to pre-emptively attack an adversary believed to be on the verge of a nuclear breakthrough. They contended that intelligence indicated Iran was in the final stages of assembling a nuclear device, thus presenting a 'last window of opportunity' to act, even absent an immediate, active attack. The specifics of this intelligence remain largely undisclosed by Israel.

Yet, this justification triggered a heated debate and an immediate pushback from international law experts and arms control monitors. ‘These attacks represent a blatant act of aggression and a violation of jus cogens norms – peremptory rules from which no derogation is permitted,’ a group of UN human rights experts said in a statement citing the UN Charter’s prohibition on the initiation of use of force. The experts noted that 'there is no evidence whatsoever that Iran intends to imminently attack the US or Israel with a nuclear weapon.'

The International Commission of Jurists says Israel’s use of armed force violated Iran’s sovereignty and territorial integrity, posing a major threat to international peace and security. The core legal debate around both US and Israeli strikes hinges on interpreting and applying Article 2(4) and Article 51 of the UN Charter, which govern the ban on the use of force and the right to self-defence. ‘Pre-emptive strikes are always a violation of international law unless authorised by the UN Security Council,’ says Mary Ellen O'Connell of the Law School at the University of Notre Dame, Indiana. ‘That obviates attacking in anticipation of or to pre-empt an attack that is not yet occurring.’ She says the strikes on Iran did not constitute self-defence nor possess such authorisation.

Some legal experts defended the strikes on grounds that a status of conflict had already existed between Israel and Iran and that waiting for an attack on Israel to materialise could be catastrophic and therefore the strikes were justified. O'Connell, however, counters that international law defines armed conflict as an exchange of organised armed fighting, a condition absent when Israel attacked on 13 June. ‘The US joined the fighting and attacked a nuclear site. It was acting collectively with Israel but equally had no right to resort to force. International humanitarian law did apply at that point but would also prohibit the attack on the nuclear site for several reasons,’ she explains. ‘The nuclear site had no role in the fighting between Israel and Iran. It was well known that no weapons existed there or would exist there for months or years.’

Federica D'Alessandra, Co-Chair of the IBA’s Rule of Law Forum, explains that assessing the legality of such actions hinges on whether Iran was actively assembling a nuclear weapon at that moment, justifying a ‘last window’ response. ‘International law does however permit the use of force in “anticipatory self-defense”, but under strict conditions of imminence – alongside respect for the jus ad bellum principles of necessity and proportionality,’ she says.

Conflicting intelligence, including US assessments from March, complicates a definitive conclusion, leading to divided expert opinions as full justification rests heavily on Israel's undisclosed intelligence. ‘It’s important to underscore that legality of the use of force does, in large part, depend on whether the threat was assessed as imminent, contextually or temporally, which Israel has said it was,’ says D'Alessandra. ‘However, the burden does fall on the state using force to demonstrate the imminence of the threat and to advance sufficient information to support its legal justification.’

As for nuclear sites, D'Alessandra says that under jus in bello, which addresses the lawful use of force during hostilities, striking facilities used purely for civilian purposes would be illegal. Conversely, if those facilities were ‘dual-use’; meaning they were being employed for the production of a nuclear weapon, in violation of Iran's non-proliferation obligations, and the threat was deemed imminent, then they would constitute a legitimate military target.

On the targeting of scientists or civilians, D'Alessandra said the legality of striking nuclear scientists depends on whether they directly participated in hostilities. This requires a case-by-case assessment of their work's direct contribution to military action and its significant threat to the opposing party. 'If Iran was actively pursuing a nuclear weapon, and those scientists’ work was essential to the weapon production, then they would have constituted lawful military targets as they would have been considered as actively involved in hostilities based on their conduct, thus losing protections associated with their civilian status,' she says, 'otherwise, their targeting would have been unlawful.'

French President Emmanuel Macron acknowledged that the US strikes lacked a legal framework, but nevertheless said there was ‘a legitimacy in neutralising Iran's nuclear structures’. A joint statement by France, Germany and the United Kingdom similarly justified the US attacks. Critics contend these statements represent a political justification of the objective, rather than a legal defence of the means used.

‘The attacks on Iran cannot be seen as legitimate,’ says the Stockholm International Peace Research Institute, an independent Swedish think tank. The group highlighted the civilian casualties of those attacks. ‘The attacks against Iran’s civilian nuclear sites amount to aggression that violates international law, as do the Israeli assassinations of Iranian nuclear scientists, some in their residences. International humanitarian law prohibits attacks directed against civilians and civilian objects,’ says O'Connell. ‘Only members of the armed forces and civilians directly participating in the armed conflict hostilities are lawful targets.’

D'Alessandra says it’s hard to see how media workers or a TV station could be a legitimate target for either side. ‘War propaganda does not amount to active participation in hostilities, and the targeting of journalists without a lawful military justification is a serious breach of the laws of war,’ she says. ‘Media facilities and journalists are a protected civilian category under the laws of war – based on their status – and do not lose their protection unless their activities amount to active participation in hostilities.’

The strikes lacked a UN mandate and clear self-defence justification and carried significant humanitarian and environmental risks, making their legality a subject of global condemnation. ‘Respect for international law has plainly taken another blow,’ says O'Connell, referring to the dozens who lost their lives, those who were injured and the environment that has been damaged ‘pointlessly and unlawfully’.

‘Treaties are the way to end weapons programmes. Not violence,' she says.