For the past two months, President Donald Trump and other administration officials have been threatening to attack Iran. Initially, the threats appeared to be tied to the Iranian government’s violent crackdown on anti-government protests ignited by economic hardship. In early January, for instance, Trump warned that if Iran “violently kills peaceful protesters, which is their custom, the United States of America will come to their rescue.” He added, “We are locked and loaded and ready to go.” Two weeks later, with U.S. forces moving into the region, Trump posted, “Iranian Patriots, KEEP PROTESTING – TAKE OVER YOUR INSTITUTIONS!!!… HELP IS ON ITS WAY.” That help never came, with human rights activists reporting more than 7,000 deaths since the protests began.
The administration’s threats have continued unabated, but their purported rationale has shifted to shutting down the Iranian nuclear program, presumably based on a belief that Iran is rebuilding nuclear facilities that Israel and the United States attacked in June of last year. By late January, Trump was demanding that Iran “come to the table” to negotiate a “fair and equitable deal—NO NUCLEAR WEAPONS.” He warned that if Iran did not, the next attack “will be far worse” than the previous one.
This month, negotiations between Iran and the United States on the nuclear question resumed after being interrupted by the June 2025 attacks and the Iranian kinetic response against both Israel and U.S. military bases in the region. With the talks off to a slow start, Trump doubled down. On Feb. 19, speaking to the so-called “Board of Peace,” he set a 10-15-day deadline to reach a deal, declaring that “[o]therwise bad things will occur.” At the same time, Trump asserted that Iran’s nuclear capability had been “decimated” in the earlier U.S. strikes, but nevertheless warned that “we may have to take it a step further.” Most recently, reports have emerged that he is considering a limited strike to compel Iran to agree to U.S. terms. And the United States has now deployed extensive military forces into the region capable of carrying out either that threat, or the threat of a “potentially weeks-long operation.”
Iran has responded to the U.S. threats in a letter to the U.N. Secretary-General claiming that it will “respond decisively and proportionately” in self-defense if attacked, pointing out that “all bases, facilities, and assets of the hostile force in the region would constitute legitimate targets.” Importantly, the United Kingdom has blocked the use of its air bases in the U.K. (RAF Fairford) and Diego Garcia for U.S strikes.
In this Q&A, we examine the international law issues raised by the U.S. threats and potential strikes against Iran. In short, any U.S. military strike on Iran under the current circumstances — either in relation to the dire human rights situation within Iran or its damaged nuclear program — would (once again) violate international law’s bedrock prohibition on the use of force. Any use of force by the United States against Iran would also trigger an international armed conflict to which all four Geneva Conventions and other rules applicable in armed conflict would apply. Other States are legally bound not to support or assist in any U.S. military action against Iran, and any potential agreement reached on Iran’s nuclear program under credible threat of U.S. force could be considered legally void. (To understand the legality of military action against Iran under U.S. domestic law, see this Q&A by one of us, Tess, and Brian Egan from last summer.)
1. Would the United States be entitled under international law to use force against Iran to stop its crackdown on the protesters?
No. The legal issue is whether one State may forcibly intervene in another State to stop the latter from killing people (or otherwise committing atrocities) located on the latter’s territory, known as “humanitarian intervention.” The prevailing view among States is that such a use of force would violate the U.N. Charter’s Article 2(4) prohibition on the use of force, absent Security Council authorization (e.g., Somalia (1992 and 1993), Sudan (2005 and 2006), and Libya (2011)). As the International Court of Justice stated in its 1986 Paramilitary Activities judgment, “the use of force could not be the appropriate method to monitor or ensure such respect” for human rights (¶ 286). Accordingly, a U.S. military operation against Iran on the basis of humanitarian intervention would be unlawful as such, even in the face of Iran’s condemnable, deadly suppression of the recent protests.
Since that judgment, however, support for a “humanitarian interventions” exception to the prohibition on the use of force has grown (see here). In particular, the United Kingdom now takes the position that humanitarian intervention is lawful when “there is convincing evidence, generally accepted by the international community as a whole, of extreme humanitarian distress on a large scale, requiring immediate and urgent relief.” In such cases, there must be no alternative to using force to save lives. The proposed use of force must also be “necessary and proportionate to the aim of relief of humanitarian suffering and must be strictly limited in time and in scope to this aim (i.e. the minimum necessary to achieve that end and for no other purpose).”
The position that there is a customary international law norm permitting humanitarian intervention remains an outlier, and many States, scholars, and analysts have noted the slippery slope and the susceptibility to abuse it would engender if accepted more widely. But even if one adopts this approach, a U.S. military operation against Iran would not pass muster. While a number of the factors typically considered in a humanitarian intervention analysis would seem to be lacking, the most significant here is that humanitarian intervention is permissible only when the purpose is to stop human suffering. The very risk that humanitarian intervention may be a subterfuge for interference in another State is what renders the doctrine unacceptable to many States. And here, that risk is all too real: it is doubtful that the Trump administration truly harbored humanitarian motivations, especially since the claimed concern for the protesters faded so quickly, only to be replaced by pressure to accept U.S. demands in nuclear-related negotiations.
2. Were the U.S. threats to use force in support of the protesters lawful?
No. Article 2(4) of the U.N. Charter prohibits not only the use of force but also the threat to use it. Of course, not every such threat is unlawful. To be unlawful (as one of us, Mike, has explained), a threat by one State to use force against another must be communicated effectively to the latter and be coercive. To be coercive, it must also be credible. Most importantly, there must be no legal basis for the threatened use of force, such as self-defense.
That is exactly the case here. Trump and other U.S. officials have issued threats of force publicly, and, considering the significant U.S. forces that have moved into the area, the United States is clearly capable of carrying them out. The threats are particularly credible given the recent U.S. willingness to use force against Venezuela, as well as the fact that it conducted significant strikes against Iran itself less than a year ago. Most significantly, as explained above, there is no basis in international law for U.S. military action against Iran on purported humanitarian grounds. Therefore, the United States has breached its obligation under Article 2(4) to refrain from unlawfully threatening another State with military force.
3. Would the United States be entitled to use force against Iran based on the threat posed by the Iranian nuclear program?
No, not based on the present facts. Article 51 of the U.N. Charter provides that, despite the prohibition on the use of force in Article 2(4), a State may lawfully use force to defend itself against an “armed attack.” It is universally accepted that the use of force in self-defense must comply with the conditions of both necessity and proportionality. Necessity limits the right to use force defensively to situations in which non-forcible measures cannot address the armed attack. The necessity criterion also includes a temporal factor according to which a defensive use of force in anticipation of an armed attack is permissible only when that armed attack is “imminent.” When it is, and the other requirements of necessity and proportionality are satisfied, so-called “anticipatory self-defense” is permissible.
Although there is some disagreement about how intense a use of force must be to qualify as an “armed attack” triggering the right of self-defense, a nuclear strike by Iran would self-evidently do so. Yet, a U.S. strike would fall short of the necessity requirement on two grounds. First, there is no reason to believe an Iranian nuclear attack is imminent. Iran currently does not have the capacity to launch one (it has no nuclear weapons) and has expressed no intent to attack. Indeed, as mentioned in the introduction, Trump has stated that the U.S. strikes in June “decimated” the Iranian nuclear capability. To assume that the rebuilding effort has been successful to a level where Iran can imminently launch an attack against the United States, and has formed the intent to do so, is beyond credulity.
Second, negotiations are underway, so there is, at present, a viable non-forcible alternative to the use of force. The very fact that they are ongoing means that the United States itself has not concluded that they will prove futile. And even if it claimed negotiations were doomed to failure and cut them off, that claim would have to be assessed against a reasonableness standard. Current reports indicate the Iranians are negotiating seriously, even if not entirely to the United States’ satisfaction. Most important on this score, failed talks (assuming a genuine impasse rather than a pretense of diplomacy) do not, by themselves, give rise to the necessity of using force in self-defense. Should talks fail — as is indeed plausible — Iran would still need to have a nuclear weapons capability and a clear intent to use it against the United States for U.S. strikes in self-defense to be lawful, as described above.
It is also worth remembering that merely having a weapons capability is not a ground for invoking self-defense. The United States possesses an arsenal of nuclear weapons capable of striking potential adversaries anywhere across the globe, but that capability alone has not been — and should not be — considered a threat of armed attack, let alone one that could satisfy the necessity requirement of anticipatory self-defense.
There are quite simply no grounds for justifying an attack against Iran based on anticipatory self-defense of the United States.
4. Are the threats to use force lawful if Iran does not agree to get rid of its nuclear program?
No. The same criteria that apply to the U.S. threats to use force based on humanitarian intervention (see above) apply equally to the threat to use force to prevent Iran from acquiring a nuclear weapons capability. All of the criteria for a violation of the prohibition on the use of force have been satisfied by the U.S. threats. They were very publicly made, the presence of U.S. forces in the region makes them credible, the willingness of the United States to employ force against other States has recently been demonstrated — including against Iran less than a year ago — and, as explained in the previous question, no reasonable argument can be fashioned that the United States has a right of self-defense against an imminent or ongoing Iranian nuclear attack.
As with the threat to use force to stop Iranian suppression of protests, the U.S. threats to use force to deprive Iran of a nuclear weapons capability, in the current circumstances, were, and remain, unlawful.
5. But can the United States use force to defend Israel against an Iranian attack?
No. International law allows States to use force in the defense of another State when the latter has requested assistance, subject to two conditions. First, there must be an ongoing or imminent armed attack against the former that triggers a right to necessary and proportionate forcible self-defense. Second, that State must first request assistance, and the assisting State’s actions are strictly limited to the four corners of that request.
In this case, it is reasonable to assume that Israel’s current government would be willing to ask the United States to strike Iran and would place very few restrictions on the U.S. operations. However, in our view, there is no basis for Israel to strike Iran at this time, and therefore, there is no derivative right of the United States to engage in collective self-defense.
In this regard, it has been suggested that either self-defense justified Israel’s June strikes on Iran, Operation Rising Lion, or they were mounted against military objectives in an ongoing international armed conflict with Iran (see, Mike’s analyses of the arguments here). Therefore, the argument would go, the United States had the right to follow up the Israeli attack with its own strikes, Operation Midnight Hammer (see Mike’s analysis of that argument here). Two of us (Mike and Tess) have argued that these are not convincing propositions (see Mike’s pieces just cited and Tess’s discussing why there was arguably no viable self-defense claim to be made; see also this analysis arguing that the United States did not make a convincing case for collective self-defense at the time of those strikes; for a contrary view see this analysis by Cohen and Shany).
Regardless, even on its own terms, the theory that the June strikes were justified as part of an “ongoing international armed conflict” (or ongoing armed attacks by Iran and attributable to Iran via its proxies) would not apply to U.S. strikes in February 2026. The underlying premise of ongoing hostilities – even if it were to have been sufficient at the time – no longer holds. What’s more, the theory, as articulated best by Cohen and Shany, also requires a fresh analysis of the resort to force during the ongoing conflict, and as explained above, U.S. strikes in the present day would not satisfy requirements of necessity and proportionality.
In any event, the facts on the ground have changed since June. Given the degree of damage caused during the June attacks, the negotiations over the nuclear program presently underway, the potential deterrent effect of Israel’s and the United States’ demonstrated willingness to strike Iran if they perceive a threat, and an unquestionable operational capacity to do so with relative impunity, Israel cannot craft a colorable argument that it must act now in anticipatory self-defense to fend off an imminent armed attack. That being so, there is no corresponding right of collective self-defense on the part of the United States.
6. Did international law require the United Kingdom to refuse to allow the United States to use its air bases for strikes on Iran?
Yes. As explained above, any use of force by the United States against Iran in the current circumstances would be a violation of international law. Under the law of State responsibility:
“A State which aids or assists another State in the commission of an internationally wrongful act by the latter is internationally responsible for doing so if: (a) that State does so with knowledge of the circumstances of the internationally wrongful act; and (b) the act would be internationally wrongful if committed by that State” (Articles of State Responsibility, art. 16).
In this case, a U.S. attack would violate the prohibition on the use of force in Article 2(4). That same obligation to refrain from the use of force binds the United Kingdom (and all other States, as it is a binding rule of customary international law). The circumstances surrounding any use of force by the United States are widely known because the United States has publicly announced its willingness to use military force against Iran. As a result, if the United Kingdom allowed the United States to use its air bases for the strikes, it would be responsible under international law for its contribution to the unlawful U.S. operation. Accordingly, it had no legal option but to make clear that the United States could not use the bases for a strike on Iran.
7. What legal effect would the U.S. threats to use force have on an agreement between the United States and Iran on its nuclear program?
It could render an agreement void. As we explained in regard to U.S. threats to forcibly acquire Greenland, Article 52 of the Vienna Convention on the Law of Treaties (this provision of the VCLT is recognized as reflecting binding customary international law) provides that a “treaty is void if its conclusion has been procured by the threat or use of force in violation of the principles of international law embodied in the Charter of the United Nations.” This provision has traditionally been interpreted as a very limited basis for finding a treaty void. The mere fact that there is a potential threat of the use of force or significant asymmetry in the Parties’ military capabilities does not render an agreement void. Instead, the duress must effectively deny the threatened Party meaningful choice regarding the instrument and its material terms. But if that threshold is reached, the treaty is void, ab initio, not merely voidable by the Party that was under duress.
That high threshold is arguably met here. There is an ongoing U.S. threat to unlawfully use force that Trump himself directly ties to Iran’s rejection of terms that are favorable to the United States in the current negotiations. Given the capacity of U.S. forces in the region (as well as those that can strike from afar), the United States can deliver a devastating blow against Iran at will, a highly credible possibility given prior U.S. strikes on Iran and the administration’s recent willingness to use military force in violation of international law. It is difficult to imagine how the situation could be characterized as anything other than duress.
It is likely, however, that the United States and Iran may be negotiating a text that takes the form of non-binding political commitments rather than a binding agreement governed by international law (including the VCLT). The previous Joint Comprehensive Plan of Action (JCPOA) was an example of a non-binding arrangement on Iran’s nuclear program, though it was also backed by a binding Security Council resolution. If that were to be the result of the current negotiations, the formal rules of treaty interpretation would not render any arrangement that is reached void as a matter of law. Still, the credible U.S. threats would nevertheless cast grave doubt on the legitimacy and durability of the arrangement, given that Iran would have a highly credible argument that it was reached under duress.
Editors’ Note: Readers may also be interested in Brian Egan and Tess Bridgeman, Top Experts’ Backgrounder: Military Action Against Iran and US Domestic Law, Just Security (June 18, 2025)
FEATURED IMAGE: In this handout photo obtained from the U.S. Defence Department, Nimitz-class aircraft carrier USS Abraham Lincoln (CVN 72) sails alongside Arleigh Burke-class guided-missile destroyer USS Frank E. Petersen Jr. (DDG 121) and Lewis and Clark-class dry cargo ship USNS Carl Brashear (T-AKE 7) in the Arabian Sea, Feb. 6, 2026. (Handout photo by Mass Communication Specialist 1st Class Jesse Monford/U.S. Navy via Getty Images)
Great Job Michael Schmitt & the Team @ Just Security for sharing this story.
