The UN Charter ...
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Article 51 recognizes the right of self-defense in the event of an armed attack. To treat a potential future threat as grounds for military action would be to accept preventive war based on the unilateral judgment of individual states. Such an interpretation would seriously undermine, if not altogether empty, the prohibition on the use of force of its practical meaning.
The legality of the US attack has also been challenged within the American legal community. In a statement issued on March 6, 2026, the president of the American Branch of the International Law Association described the attack on Iran as unlawful, stressing that speculative threats concerning what might happen in the future cannot justify the use of military force.
The attacks on schools, sports stadiums, civilian infrastructure, and Iranian nuclear facilities subject to International Atomic Energy Agency safeguards raise further questions under international law. Apart from the prohibition on the use of force, these attacks must also be examined in light of the rules of international humanitarian law. Parties to an armed conflict are required to distinguish between military targets and civilians, refrain from disproportionate attacks, and take all feasible precautions to minimize harm to civilians and civilian infrastructure.
Military aggression carries international legal responsibility
UN General Assembly Resolution 3314, which sets out the Definition of Aggression, identifies acts such as attacks by one state’s armed forces against the territory of another and the blockade of a state’s ports or coasts as acts of aggression. In the absence of a valid legal justification for the use of force, the US military attack on Iran constitutes an internationally wrongful act for which the United States bears responsibility.
Under the law of state responsibility, a state incurs international responsibility when conduct attributable to it breaches an international obligation. That responsibility entails specific legal consequences: the state must cease the wrongful conduct if it is continuing, offer appropriate assurances and guarantees that it will not be repeated, and make full reparation for the injury caused. Political or security considerations cannot, by themselves, relieve the United States of the legal consequences of its actions.
Serious breaches of obligations arising under peremptory norms of international law, or jus cogens, also have implications for other states. International law requires states to cooperate through lawful means to bring such breaches to an end, refrain from recognizing as lawful situations created by them, and withhold aid or assistance that would help maintain those situations.
Iran, for its part, retains the right of self-defense under Article 51 of the UN Charter in response to an armed attack. It may exercise that right subject to the conditions and limitations imposed by international law.
This is about more than Iran
The significance of this case extends far beyond Iran’s security or the balance of power in the Middle East. What is ultimately at stake is the credibility of an international legal order designed to prevent states from resorting to force at will. If military strength is allowed to take precedence over legal obligations, and states responsible for aggression can escape accountability, no country’s security can be taken for granted.
The prohibition on the use of force is not a concession that powerful states make to weaker ones. It is a binding obligation that applies equally to all states. Ending US aggression, respecting Iran’s sovereignty and territorial integrity, and providing reparation for the damage caused by unlawful military action are matters of legal obligation, not political goodwill.
If power is permitted to prevail over law, Iran will not be the only casualty. The foundations of the international legal order, and the authority of the UN Charter itself, will also be at risk.
The article first appeared on Kyodo News on November 13.