Briefing note on the Legality of Military Action by Israel and the US against Iran

This note on the legality of military action by Israel and the US against Iran, is based on a UKLFI Charitable Trust webinar with Natasha Hausdorff and Hillel Neuer held on 7 March 2026.

Key Takeaways

Recent public statements asserting that military operations by Israel and the United States against the Islamic Republic of Iran violate international law rely on a mischaracterisation of the applicable legal framework.
Many of these related assessments analyse the operations as a new resort to force under the framework of jus ad bellum. This overlooks the reality of the ongoing armed conflict involving Iran and Israel, which has lasted decades. Here the applicable legal framework is that of jus in bello, the body of international humanitarian law governing the conduct of hostilities, including the principles of military necessity, distinction, proportionality and precaution.
A state such as the United States may assist Israel, as an ally, in such ongoing armed conflict, but it is also highly relevant to consider the sustained hostile actions by the Iranian regime against the United States and its interests over several decades.
Under international law, states retain the inherent right of self-defence, recognised under Article 51 of the United Nations Charter, which may be exercised individually or collectively.
Persistent misstatements of international law risk eroding confidence in the international legal order and distorting diplomatic deliberations within the United Nations system.
In addition to the framework of ongoing armed conflict and collective self-defence, the doctrine of anticipatory self-defence against an imminent threat may also be considered, particularly in light of repeated US statements over the concern of imminent Iranian development of nuclear weapons capabilities and long-range missile systems, such as intercontinental ballistic missiles (ICBMs).

Situational Assessment

The Islamic Republic of Iran has conducted sustained hostile actions against Israel, including through proxy forces that it arms, finances and directs, notably Hezbollah, Hamas and other militias operating across the region.
Under established principles of state responsibility, states are responsible for the acts of armed groups that they organise, direct, support or substantially assist.
Iran has conducted direct ballistic missile and drone attacks against Israeli civilian communities, while continuing to expand its ballistic missile arsenal and nuclear programme and persistently threaten Israel with destruction.
The regime has also engaged in hostile actions against the United States, including attacks against American personnel, citizens and national security interests, as well as proxy attacks that have resulted in the deaths of hundreds of US service members.
In this context, the current military operations conducted by Israel and the United States should properly be understood as actions undertaken in the context of an ongoing armed conflict, rather than the initiation of a new set of hostilities.
Assertions that such operations require prior authorisation by the UN Security Council misunderstand and misrepresent the international legal framework. States do not require UN approval to exercise their inherent right of self-defence or to engage in an ongoing armed conflict.
Under the laws of armed conflict, civilian casualties resulting from military operations do not automatically constitute violations of international law. The legal analysis focuses on intent, rather than effect, and is based upon the main principles of military necessity, distinction, proportionality and precaution. The prospect of harm to civilians is unfortunately inevitable in any armed conflict, and will often be unavoidably higher in urban populated areas. The aim of the laws of armed conflict is to mitigate civilian and collateral damage so far as feasible.

Diplomatic Implications and Recommendations

Democratic Member States should ensure that diplomatic discussions concerning the legality of current military operations are grounded in the correct legal analysis governing ongoing armed conflicts, rather than mischaracterised as a new resort to force.
Member States should reaffirm that Article 51 of the United Nations Charter recognises the inherent right of states to act in individual and collective self-defence, including in response to sustained armed attacks and credible threats.
Diplomatic interventions at the United Nations should consider the full strategic and legal context of Iranian aggression, including decades of proxy warfare, direct missile attacks, terrorism and threats directed at Israel, the United States and other Member States.
Member States should remain vigilant regarding the spread of misinformation concerning international humanitarian law, particularly narratives that portray lawful defensive operations as inherently unlawful.
Upholding the credibility of the international legal system requires that international law be applied consistently, accurately and in full context, rather than selectively invoked for political purposes.

The full webinar can be viewed at https://www.youtube.com/watch?v=3kWEwrVez94 or https://www.youtube.com/watch?v=Pe2VUwExjwo

With particular thanks to Daniel Radomski at UN Watch for preparing the draft of this note