In international law, is Israel's use of force against Iran justified by self-defence? - ABC Religion & Ethics

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On 12 June 2025, Israel launched a major military operation against Iran targeting its nuclear programme — including facilities, individual scientists and military leadership. I want here to inquire into the legality of Israel's use of force against Iran as a matter of the jus ad bellum . As I will explain, Israel's use of force against Iran is, on the facts as we know them, almost certainly illegal.
The only justification that Israel can provide for its use of force is self-defence under Article 51 of the UN Charter — which legitimises the use force to repel an armed attack, subject to the criteria of necessity and proportionality.
The first point to clarify here is that the nature and stated goals of Israel's use of force — damaging Iran's nuclear programme and preventing it from developing a nuclear weapon — are explicitly about deflecting a future armed attack by Iran against Israel. Which is to say, it seeks to prevent an attack that is yet to occur. In other words, this is not a situation in which Israel is allegedly responding to an Iranian attack occurring now , whether directly or through proxies such as the Houthis in Yemen or Hezbollah in Lebanon. Prior uses of force between these two states can be legally and factually relevant as context, but the issue here is squarely about stopping a future nuclear attack by Iran.
Assessing the legality Israel's use of force against Iran thus depends on the legal conception of self-defence being applied and on the facts to which the legal rules can be applied. There are three possible legal positions (with some variations on a spectrum) on uses of force in response to future armed attacks:
1. That states can act preventively to deflect threats, especially existential threats. 2. That states can act to deflect future armed attacks that are imminent. 3. That states can only act to deflect armed attacks that have occurred.
If one regards the third position as correct, then Israel's use of force against Iran would be manifestly illegal. But, while it is difficult to reliably establish what the majority view on some of these issues is, I think we can say that reasonable scholars and states have argued that the third position is too restrictive.
We can easily say, however, that there is unanimous agreement among international lawyers that the first position is legally untenable. It is associated, for example, with some of the arguments that lawyers in the Bush administration used to justify force against Iraq — that it might give weapons of mass destruction to terrorist actors who might use them against the United States, and that the threat is so grave that the United States would be defending itself from Iraq. It is also similar to arguments that Russia has used to justify its invasion of Ukraine — that after joining NATO, Ukraine might attack Russia at some future point. The facts of these two examples aside, the problem with this approach is that it is so boundless that it completely eviscerates the prohibition on the use of force: a state could act whenever it perceives an existential threat. After all, Russia, the United States and China have the ability to destroy each other within hours, but that kind of capability cannot automatically mean that they can start a war and call it 'self-defence'.
In short, this 'preventive' form of self-defence is simply not self-defence at all. With regard to Iraq, even the UK — America's closest ally — expressly disavowed such a legal theory, and the United States itself did not formally rely on this argument internationally.
Iranian Supreme Leader Ayatollah Ali Khamanei attends an event in Tehran, Iran, on 10 May 2025. (Photo by IRANIAN LEADER PRESS OFFICE / Anadolu via Getty Images)
Therefore, Israel could only justify its use for against Iran by relying on the second, intermediate position — that Iran's nuclear attack on Israel was imminent . The question of anticipatory self-defence against imminent attacks has been much debated.
Broadly speaking, there are two theories of what constitutes an 'imminent' attack is. The first, restrictive one is temporal in character: an imminent attack is one that is about to happen. The second, more expansive understanding is causal in character: an imminent attack is one where the state concerned will obtain the capability to conduct the attack and intends (has irrevocably committed itself) to the attack at some point in the future. This more expansive approach is often coupled with the necessity of an immediate response — that now is the 'last possible window of opportunity' to act to stop the attack.
On the temporal understanding of an imminent attack, there is simply no plausible way of arguing that Iran was about to attack Israel with a nuclear weapon, which it doesn't even possess. Thus, the only plausible legal theory of self-defence that Israel could invoke would be the causal, non-temporal one. Let's assume, for the sake of the argument, that this theory is correct. If so, Israel could justify its use of force only if the following two propositions of fact were both true:
1. That Iran's leadership has irrevocably committed itself to attacking Israel with a nuclear weapon, once such a weapon has been developed — which is to say, it intends to attack Israel in the future once it has the capability to do so. 2. That today was the last window of opportunity to stop the attack from happening — it was necessary to act now and no non-forcible alternative could have removed either Iran's future capability or its intention to attack Israel.
Now, obviously, I am not privy to the information that, say, Israeli, American or British intelligence services have about the intentions of Iran's leadership. Working solely from information that is publicly available, I would say that the situation as it stands is as follows:
There are many public statements by various Iranian officials over the years calling, expressly or impliedly, for Israel's destruction.
There are many public statements by various Iranian officials over the years calling, expressly or impliedly, for Israel's destruction. That kind of rhetoric, however, is mainly meant for domestic political purposes in Iran and does not, in itself, directly translate to an intention of the Iranian leadership to use a nuclear weapon against Israel, despite the fact that Israel itself already has nuclear weapons and could retaliate against Iran decisively.
That kind of rhetoric, however, is mainly meant for domestic political purposes in Iran and does not, in itself, directly translate to an intention of the Iranian leadership to use a nuclear weapon against Israel, despite the fact that Israel itself already has nuclear weapons and could retaliate against Iran decisively. Maybe Israeli officials do have some intelligence about the intentions of Iran's leaders — especially Ayatollah Ali Khamenei — to attack Israel with nuclear weapons, but they have not provided the public with any such intelligence.
Maybe Israeli officials do have some intelligence about the intentions of Iran's leaders — especially Ayatollah Ali Khamenei — to attack Israel with nuclear weapons, but they have not provided the public with any such intelligence. This means that the first prong of the test above fails — there is little evidence, as things stand, that Iran intends to use a nuclear weapon against Israel once it obtains the capability to do so. Inflammatory, even genocidal, public statements by some Iranian officials do not suffice on their own, because they are not sufficiently probative about the intent of those Iranian officials who actually make the relevant decisions.
This means that the first prong of the test above fails — there is little evidence, as things stand, that Iran intends to use a nuclear weapon against Israel once it obtains the capability to do so. Inflammatory, even genocidal, public statements by some Iranian officials do not suffice on their own, because they are not sufficiently probative about the intent of those Iranian officials who actually make the relevant decisions. Even according to Israeli Prime Minister Benjamin Netanyahu, Iran was months from building a bomb, while negotiations between the United States and Iran were actively taking place. (It is significant that the UN Secretary-General expressed his concern about 'Israeli attacks on nuclear installations in Iran while talks between Iran and the United States on the status of Iran's nuclear programme are underway'.)
Even according to Israeli Prime Minister Benjamin Netanyahu, Iran was months from building a bomb, while negotiations between the United States and Iran were actively taking place. (It is significant that the UN Secretary-General expressed his concern about 'Israeli attacks on nuclear installations in Iran while talks between Iran and the United States on the status of Iran's nuclear programme are underway'.) Thus, while militarily Israel undoubtedly found itself in a convenient position to attack Iran — which was already weakened due to prior engagements between the two states — it is difficult to see how this was the 'last possible window of opportunity' to deflect a future nuclear attack. In other words, that the use of force was necessary, the only available option to stop this attack.
Even if the broadest possible (legally plausible) understanding of anticipatory self-defence was taken as a correct, Israel's use of force against Iran would be illegal. This is because there is little evidence that Iran has irrevocably committed itself to attacking Israel with a nuclear weapon, once it develops this capability. And even if such an intention was assumed — again, it would be for Israel to provide any further evidence of such intention — I don't see how it could plausibly be argued that using force today was the only option available.
I have limited myself here to an ad bellum analysis. In short, unless Israel is able to provide substantially more compelling evidence than is currently available to the public, it cannot reasonably be argued that Iran would imminently attack Israel, or that using force was the only option to stop that attack. Israel is therefore using force against Iran unlawfully, in violation of Article 2(4) of the UN Charter. Which is to say, it is committing an act of aggression.
There are also in bello issues to be discussed. One point that I found particularly problematic is the apparent direction of attacks specifically against Iranian scientists working for the nuclear programme. If such scientists are members of Iran's armed forces, they are combatants and can lawfully be targeted as a matter of international humanitarian law. But scientists who are civilians — and most probably are — cannot (as persons) lawfully be made the object of an attack. Simply working on a weapons programme as a researcher does not entail direct participation in hostilities that could remove civilian immunity from an attack.
To give an analogy, the hundreds of civilian scientists or engineers who worked on the Manhattan Project would not (in today's terms) be qualified as combatants or civilians taking a direct part in hostilities. The facilities in which they worked would qualify as military objectives — as would a munitions factory, for instance. But the scientists themselves as persons would not.
Marko Milanovic is Professor of Public International Law at the University of Reading School of Law, and Director of the Global Law at Reading (GLAR) research group. An earlier version of this article was published on EJIL:Talk! Blog of the European Journal of International Law, and appears here with permission.