What Are International Law Limits On the Presidents Use of Military Force?

In light of the military strikes against Iran, I thought that it might be useful to offer a basic primer on the principles of international law that govern President Trump's use of military force, and offer some thoughts on how these principles might apply to the strikes on Iran.

International law in this area largely comes from three sources: First, nations can create law by the adoption and ratification of treaties. In the context of the legality of military action, the key treaty is the U.N. Charter. Second, nations can create "customary international law" by practice. If a large number of countries declare a principle and actively act in compliance with that practice, the principle is recognized as a settled principle of international law. Third, international tribunals such as the International Court of Justice provide decisions on these issues.

In the modern era, the critical legal document is the U.N. Charter. Adopted after the horrific experience of World War II, the Charter is imposes a very restrictive view of when a nation can engage in the use of force. It provides that "All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations." The only exceptions are the use of force authorized by the Security Council and the right of individual and collective self defense. The self-defense provision, however, makes clear that this right of self-defense only applies until the Security Council has taken action:

Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.

It is hard to overstate how much the U.N. Charter constituted a break from traditional international law. Before World War I, wars for economic or territorial gain were common and acceptable. In the 1920’s, following this disastrous war, the first steps were taken to move in a different direction. Indeed, in 1928 there was even an international agreement called the Kellogg-Briand Pact, under which nations agreed promised not to use war to resolve "disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them".

While World War II made a mockery of the Kellogg-Briand Pact, its challenge to wars of aggression was placed in the U.N. Charter. My fellow Justice O’Conner law clerk, Professor Oona Hathaway of Yale Law School co-authored a great history of Kellogg-Briand and its aftermath, and makes a persuasive case that it and the U. N Charter really did change nation-state behavior.

As Zack Beauchamp noted a few years ago, “today we’re living through one of the most peaceful times in human history. This great chart from Oxford’s Max Roser — which shows the global death rate from war over the past 600-plus years — shows just how lucky we are.” The chart indeed shows a large decline ion deaths from war since the U.N. charter was adopted in the late 1940’s. This is made even clearer if we focus on the post-WW II period:

While other factors certainly contributed to this decline in military aggression—most notably the rise of nuclear weapons—the change in norms reflected in the Charter certainly played an important role. Perhaps the best evidence of the impact of the U.N. charter on nation’s behavior is the fact that even when countries use military force, they almost always offer justifications based on the Charter itself. I fully expect the Trump Administration to do the same when they release the OLC legal memo about the strikes on Iran.

The rules for use of military force can be summarized as follows: Absent approval by the Security Council, the use or threat of military force against another nation (or its territory) is unlawful unless it is done in self-defense. The scope of self-defense includes collective self-defense, which means other nations can come to the aide of another nation under attack.

The text of the U.N. Charter leaves lots of open questions such as: What is the use of force? Must a nation wait for an attack or can it take preemptive action? Practice over the years, however, has resulted in some consensus on many of the open issues, and the International Court of Justice has offered some guidance as well. For example, it is largely accepted that a nation need not wait for an an attack, but can take steps to defend itself from an attack that is imminent. There is uniform acceptance, however, that a merely preventive attack, however,--such as to take out a weapon--is not permitted unless an actually attack is imminent.

Some recent examples of military action illustrate these principles. While the Vietnam War was deeply controversial, the U.S. intervention in defense of South Vietnam was a lawful exercise of collective self defense in support of South Vietnam. Both the Gulf War and our more recent intervention in Libya were authorized by the Security Council. The legality of the invasion of Iraq in 2003 is deeply controversial, but it is worth noting that the U.S. at the time justified the war as authorized by Security Council resolutions, and in particular as the resumption of the previously authorized use of force against Iraq (to defend Kuwait) after violations of the cease fire. Finally, the fight against ISIS has the express consent of Iraq and at least the implied consent of Syria, and both are also instances of our own self-defense. The Obama Administration air campaign against Libya was also supported by Security Council resolutions.

This is not to say, however, that every military action taken by the U.S. has had even a plausible legal basis under the U.N. Charter. In recent years, the most challenging situations have been circumstances of serious humanitarian crisis in which the Security Council could not come to agreement. The best example here is our air war against Serbia to stop human rights abuses in Kosovo. The Security Council could reach no resolution, but NATO decided to act nonetheless. Vietnam's intervention in Cambodia to dislodge the Khmer Rouge is yet another example. Ironically, the legal scholars and advocates who are generally hostile to military force are often supportive of such humanitarian interventions on the basis of a “duty to protect.” Some legal scholars have used the odd phrase "illegal, but justified" to describe these cases.

Some other points that are important to note before addressing the attack on Iran. First, while the U.N. Charter Framework works well when the major powers agree that intervention is warranted, it breaks down when one major power or another uses its veto on the Security Council to prevent action. The theory of the U.N. Charter is that the Security Council is the mechanism to address security matters, and when their is an impasse leading to inaction, serious security concerns remain unaddressed. The Cambodia, Kosovo and Syrian incidents offer good examples. In each of these cases there were humanitarian disasters causing the death of hundreds of thousands and in some instances outright genocide. The Russian invasion of Ukraine is a more recent example. Yet, in each case action by the Security Council was thwarted by major power vetoes. This has lead many to question whether adopting a purist approach to the U.N. Charter on the use of force remains viable. If China and Russia will veto any humanitarian intervention, no matter how justified, perhaps a new framework will be required.

Second, the enforcement mechanism for the U.N. charter is almost entirely political. While the International Court of Justice will often rule on use of force issues, it has no enforcement authority. Similarly, the International Criminal Court has jurisdiction of the crime of aggression, it is sharply limited in its practical ability to take on cases involving aggression unless there is referral from the Security Council. (And U.S. courts have refused for years to review cases involving the use of military force by the U.S.). As a practical matter, the only enforcement is either by the Security Council itself (which happened in response to the Iraq invasion of Kuwait) or by other nations (for example, the provision of military material to Ukraine and the imposition of sanctions on Russia).

This is important because it means that the international reaction to specific instances of military force effectively creates a set of precedents for what is viewed as permissible—and what it not.

Third, it is important to note that the United States was the leading power in developing the U.N. Charter framework. It was not imposed on the U.S.; to the contrary, in large measure we (and our Allies) imposed this framework on the rest of the world. And while it has constrained our action, it appears to have restrained others aggressive action as well, and has not restrained us when our vital rights of defense were at stake--such as in the wars against terrorist groups. The result has been a far more stable world than before the Charter.

Finally, while it is often easy for a good lawyer to concoct an exception to these rules (as we did in Kosovo), we need to be cautious. An exception we come up with to suit our national interests can just as easily be used by future adversaries. Indeed, Russia has used the Kosovo example to justify its aggression in Georgia and Ukraine. Put more plainly, we need to always remember that one person's "humanitarian intervention" is another's aggression. Our examples matter.

So what about Iran? Clearly, if there is an imminent legal attack on the U.S. or its allies by Iran, we could act to defend ourselves without waiting for Iran to launch its attack. The U.S. is allowed to come to the defense of Israel. Accordingly, the focus of analysis needs to be on the Israel actions—if they are permitted self-defense, it is likely that the U.S. actions are lawful as collective defense.

As noted above, it has been uniformly understood that absent an imminent threat, using military force for prevention is not lawful. Under this view therefore, unless Iran’s development and use of nuclear weapons was “imminent”, military force is unlawful.

Those who defend the Israeli and U.S. strikes on Iran’s nuclear capabilities, however, raise an important point: when faced with a weapon that imposes an existential threat, the concept of what is imminent needs to be viewed more broadly. This was explained well by an Israeli official interviewed by the Washington Post:

“It is true there was no better time: Israelis have never been more well-practiced, and Iran and their proxies have never been weaker,” said the Israeli official. “But that’s not enough for us to operate. The reason we operated is necessity and understanding there is no alternative. What if they break out [toward a nuclear weapon] and there is no way for us to notice? There is no safety zone left.”

This is also consistent with how the Obama Administration viewed the issue while it was negotiating an agreement with Iran about Iran’s nuclear capabilities. The goal of the JCPOA was to put enough restrictions on Iran’s nuclear program so that if it decided to build a weapon, the international community would have time to respond. While intelligence assessment suggest that Iran had not yet decided to develop a nuclear weapon, these same assessments suggest that it would take little time for Iran to produce a weapon if it wanted to do so. As a CRS report explained,

According to the July 2024 ODNI report, "Tehran has the infrastructure and experience to quickly produce weapons-grade uranium." The U.S. government estimates that Iran would need as little as one week to produce enough weapons-grade HEU for one nuclear weapon, according to a State Department official in March 2022.

. . .

Until recently, the U.S. intelligence community assessed that Iran had not resumed work on its weaponization research. A State Department official told CRS in an April 2022 email that Iran would need approximately one year to complete the necessary weaponization steps. This timeline took "into consideration assessed knowledge gaps" and reflected the intelligence community's "view of Iran's fastest reasonable path to overcome them," the official added. The current assessed public timeline is unclear. Chairman of the Joint Chiefs of Staff Mark Milley testified in March 2023 that Iran would need "several months to produce an actual nuclear weapon," but he did not explain the assumptions underlying this estimate.

Notably, Israel attacked a Syrian nuclear facility in 2007 with little international reaction, and there has been a muted response to Israel’s attack on Iran, suggesting that the international community might accept an "anticipatory self-defense" justification, at least when weapons impose an existential threat.

In short, under a strict view of the international standards for use of military force, the attacks on Iran’s nuclear facilities are unlawful. The real world security realities, however, may support a looser standard in this case.