President Biden’s First Use of Force and International Law

by | Mar 1, 2021

Airstrike. F-15 Strike Eagles. U.S. Air Force photo by Staff Sgt. Sean Carnes


On February 25, President Biden authorized the first use of military force since becoming President. The operation involved two F-15s dropping seven 500-pound Joint Direct Attack Munitions (JDAM) bombs on a grouping of buildings at a Syria-Iraq border crossing. According to the DoD, “Iranian-backed militant groups, including Kait’ib Hezbollah and Kait’ib Sayyid al-Shuhada,” used the facilities as a “terrorist entry control point.” The Syrian Observatory for Human Rights reports that the airstrike—which took place at 1 AM as a weapons shipment crossed from Iraq into Syria—killed 22 militiamen.

The operation was in response to a February 15 attack against the Erbil airport in Kurdish-controlled northern Iraq. That attack resulted in the death of a Filipino contractor and an Iraqi civilian and the wounding of a Louisiana National Guard soldier and four American contractors. In response, White House spokesperson Jen Psaki warned, “the President is sending an unambiguous message that he’s going to act to protect Americans. And when threats are posed, he has the right to take an action at the time and in the manner of his choosing.”

This was not the first engagement between U.S. forces and these groups. The United States has targeted the groups in the past, blaming them for attacking U.S. personnel and interests (see also here and here). In fact, the 2020 U.S. drone strike targeting Iranian general Qassim Suleimani also killed Abu Mahdi al-Muhandis, who helped found the Kait’ib Hezbollah and was, at the time of his death, a senior commander in Iraq’s Popular Mobilization Forces. The Popular Mobilization Forces is an umbrella organization mostly comprised of Shia militia mobilized by the Iraqi government in 2014 to fight ISIS. Since the weakening of ISIS, components of the group—like Kait’ib Hezbollah and Kait’ib Sayyid al-Shuhada—have conducted operations against the coalition forces, engaged in sectarian violence, and been supporting government forces in Syria.

In international law terms, the United States has justified these strikes by reference to the right to self-defense set forth in Article 51 of the UN Charter. This justification has drawn criticism (see here and here). In my estimation, the law governing such strikes is somewhat unsettled, but a strong case can be made that the strike complied with both the law of self-defense and international humanitarian law (IHL).


According to Pentagon Press Secretary John Kirby,

Article 51 of the United Nations [Charter], international law, [ ] gives nations involved in operations the right of self-defense. And as I said at the very outset, this really was a defensive strike meant to help protect in the future American forces and coalition partners, given what we knew those structures were used for right there on the other side of that border to provide throughput for these groups and their activities inside Iraq. So very much was a defensive operation to protect our troops and our coalition partners, as well as, as I said at the outset, to send a strong signal about our resolve.

This explanation must be assessed from a number of angles (see, e.g., here and here).

Purpose. When asked about the message being conveyed by this strike, President Biden indicated it was a warning to Iran that “You can’t act with impunity. Be careful.” And as to the groups themselves, Kirby noted, “Clearly, we want there to be a deterrence message here delivered straight to them about the repercussions of going after our people and our Iraqi partners and our facilities inside Iraq.”

Military operations are conducted for many reasons, including to signal resolve and deter future attacks. There is nothing improper with an operation having such purposes, but they do not alone justify the use of force. Instead, a State may only resort to force: 1) with the approval of the Security Council under Chapter VII of the UN Charter; 2) with the consent of the State into which it is conducted; 3) to defend against an “armed attack” under Article 51 and customary international law;  and (perhaps) 4) as a humanitarian intervention.

The self-defense precondition of response to an “armed attack” is found in the text of Article 51 itself. There is some disagreement as to where the armed attack threshold lies. Still, it would seem clear that repeated attacks against U.S. forces amount to violence at that level. Indeed, it is this purpose criterion that distinguishes last week’s airstrike from the Trump administration’s 2017 response to Syria’s use of chemical weapons against the village of Khan Sheikhoun. The latter could only have been justified legally on the questionable grounds of humanitarian intervention.

Attacker. Before the attacks of 9/11, most scholars and States viewed Article 51 as applicable to armed attacks by States, or, as observed by the International Court of Justice in its Paramilitary Activities judgment, those mounted by a non-State group sent “by or on behalf of a State” or with the State’s “substantial involvement therein.” This view was not universal, but it was the prevailing one.

However, in the aftermath of the 9/11 attacks, many States, international organizations like NATO and the United Nations, and international law scholars began to treat Article 51 as encompassing armed attacks conducted by non-State groups even when they lacked the requisite relationship with a State (in that case, al Qaeda). While the International Court of Justice’s post-9/11 Wall advisory opinion and its Armed Activities judgment adopted the Paramilitary Activities requirements, the United States and many other countries continue to maintain that self-defense is available as a legal basis for responding to armed attacks by groups such as those targeted on February 25. For instance, this interpretation provided the legal basis for the 2014-2016 notifications regarding self-defense against ISIS submitted to the U.N. Security Council by the United States, United Kingdom, Turkey, Canada, France, Australia, Germany, Denmark, Norway, and Belgium. Theirs is the better view in light of the gravity and pervasiveness of the threat posed by non-State actors today. By it, the groups that were attacked by the U.S. airstrikes were appropriate objects of actions taken pursuant to the law of self-defense.

But what of the responsibility of Iran? In this regard, State Department spokesperson Ned Price warned, “We have stated before that we will hold Iran responsible for the actions of its proxies that attack Americans,” and that “many of these attacks have used Iranian-made, Iranian-supplied weapons.” Other spokespersons have echoed that sentiment.

As a matter of law, though, there does not appear to be a sufficient link (based on open sources) between Iran and the groups in question to treat their actions as an armed attack by Iran (see the Paramilitary Activities standard above) justifying defensive uses of force against Iran itself. It is unclear that Iran even can be held responsible for their actions under the law of State responsibility. That body of law would require them to have acted pursuant to the “instructions, or direction or control” of Iran (the customary law standard reflected in Article 8 of the Articles on State Responsibility), thereby enabling the United States to seek reparations from Iran. Although the groups’ action cannot be attributed to Iran as a matter of law, Iran’s support for them is likely sufficient to amount to an unlawful “intervention” by Iran itself. These issues are, unfortunately, beyond the scope of this post.

Geography. The U.S. strikes occurred in Syria without its consent. Standing alone, non-consensual uses of force by one State in the territory of another violate the latter’s sovereignty, with some commentators suggesting that such actions even amount to unlawful uses of force against the territorial State. The question posed by the February 25 operation is whether international law provides any exceptions.

By one view, sovereignty is a veil that may not be pierced except when authorized by the Security Council under its Chapter VII authority. Some of the criticism of last week’s strike into Syria is based on this position. Yet, such a view would leave States defenseless in the face of armed attacks by non-State actors operating from a State to which the armed attack cannot be attributed.

The better view is that when international law rights are at odds—as is the case here with Syria’s sovereignty right and the U.S. right to engage in self-defense—an interpretation that best preserves both is appropriate. This has led to the so-called “unwilling and unable doctrine,” by which a State that is the victim of an armed attack may use the force necessary to defend itself in the territory of another State if the latter has been afforded an opportunity to end the hostile operations from its territory and is unwilling or unable in the circumstances to do so. The defensive actions must be limited to those that are strictly necessary and must end as soon as the need for defensive action is satisfied. Many States, including the United States, have explicitly adopted this approach, or implicitly done so through their actions. Others appear to be on the fence, while some—including Syria—reject it.

The unwilling or unable doctrine is, in my estimation, sound as a matter of law. Critics often miss the fact that self-defense is a so-called “circumstance precluding wrongfulness,” one that the International Law Commission unequivocally acknowledged in Article 21 of its Articles on State Responsibility (“The wrongfulness of an act of a State is precluded if the act constitutes a lawful measure of self-defense taken in conformity with the Charter of the United Nations.”). There is no reason to conclude that this circumstance precluding the wrongfulness of defensive actions operates only where the otherwise unlawful defensive force is directed at a State that has mounted the armed attack in question. As noted in the International Law Commission’s commentary to the article, “Article 21 leaves open all issues of the effect of action in self-defense vis-à-vis third States.” Since Syrian forces were undoubtedly unwilling to put an end to the targeted group’s activities, attacking them inside Syria’s borders was, in my opinion, lawful.

Necessity and Proportionality A State is not entitled to resort to self-defense unless its actions are “necessary” and “proportionate.” The International Court of Justice has recognized these universally acknowledged criteria in such cases as the Nuclear Weapons advisory opinion. They further appear in guidance issued to military forces, as in the DoD Law War Manual (sec 1-11). That the Biden administration accepts them is evidenced by a National Security Council spokesperson’s assertion that the February 25 strike was “necessary to address the threat and proportionate to the prior attacks.”

Necessity has both a substantive and a temporal element. Substantively, it requires that there be no alternative to the use of force to repel the armed attack. When dealing with organized armed groups in areas that the State does not fully control, the need to resort to defensive force will generally be self-evident—as it was here. Interestingly, the media questioned the selection of targets, specifically whether they had any relation to the Erbil airport attack. Doing so raised a legal red herring. Necessity merely requires that the defensive action contribute to defeating the ongoing campaign (a notion discussed below) against U.S. forces and assets. Diminishing the targeted group’s operational capabilities clearly does so.

The temporal element poses a more significant challenge. Defensive actions may neither be taken too early (imminency condition) nor too late (immediacy condition). As the airstrike responded to the Erbil airport attack, only the latter requirement is relevant.

Failure to comply with the immediacy condition is the most compelling basis of criticism leveled against the February 25 operation. For instance, commenting on Psaki’s assertion that “when threats are posed, [the President] has the right to take an action at the time and in the manner of his choosing,” one critic has observed, “That is not how international law works.” He is right. Choosing to respond when an armed attack is definitively over is mere retaliation, which is unlawful.  Therefore, he asserts, “The February 15 attack was clearly over and not ongoing. And ongoing threats are not imminent attacks.”

However, his reading of the law would denude the right of self-defense of much of its utility in situations where the victim State reasonably concludes that further attacks are likely but is uncertain of when or where they will occur. The better interpretation of the rule considers whether an individual attack is but one in a series by the same actor. If so, the attacks taken together must be regarded as a campaign. In such cases, the question is not whether a subsequent attack is imminent but whether the campaign is still underway. So long as the United States reasonably concluded in good faith that the groups would continue to mount attacks, the February 25 operation fulfilled the temporal condition.

I would go further and suggest that the groups’ prior attacks triggered the right of self-defense and that the U.S. operations responded to that ongoing campaign. The attack on the Erbil airport merely confirmed that the campaign was still underway, thereby satisfying the temporal element of self-defense.

Regarding proportionality, the National Security Council spokesperson’s assertion above that the strikes were “proportionate to the prior attacks,” which was also made by Psaki, misapplied the legal notion of proportionality. In the law of self-defense, proportionality is not about equivalency between the harm suffered by the victim State and that imposed on the attacker. Instead, proportionality in the self-defense context requires that the amount of force relied upon to repel the armed attack be no more than needed in the circumstances. This may be more than suffered by the victim State or less. In the case of the February 25 operation, the condition is satisfied, for it is even uncertain that this single strike will deprive the groups of their capability or will to mount further attacks against U.S. forces and assets.

International Humanitarian Law

The law of self-defense determines whether the United States had the right to resort to force against the targeted groups and whether it could exercise the right in Syria. But military operations conducted pursuant to the right of self-defense must also comply with other bodies of law such as IHL. While the law of self-defense determines whether an operation may be mounted, IHL governs how it may be conducted during an armed conflict.

The threshold question in any IHL assessment is whether IHL applies at all. There are two prongs to this analysis. The first is whether an armed conflict is underway. Here the United States is involved in a “non-international armed conflict” (NIAC) against “organized armed groups.” As noted by the International Criminal Tribunal for the former Yugoslavia in its Tadić  Trial Chamber judgment, the existence of such a conflict requires hostilities between a State and an organized armed group that are of sufficient intensity to distinguish them from civil disorder, such as riots. In overly simplified terms, violence must resemble a “war” between a State and a group that is both armed and well-organized (or between two or more of such groups).

As the Shia militia groups concerned have engaged in multiple attacks against U.S. forces, which have responded in kind, this condition is met. It must be cautioned that the exchanges are not occurring in the context of a non-international armed conflict between Iraq and these groups, as is the case with numerous other U.S. operations in the theater. On the contrary, the targeted groups appear to often operate with Iraq’s acquiescence. Thus, this NIAC is a separate one (or more than one NIAC with different groups) that must be assessed on its own merits.

The second prong concerns where IHL governs military operations. There are varying views on the matter. Some assert that IHL only applies in the territory of the State fighting the organized armed group. The ICRC takes a broader perspective that includes areas in neighboring States into which fighting “spills over.” At first glance, this would apply to the U.S. strike, which occurred just across Iraq’s border into Syria. However, as the U.S. operations were not in support of any Iraqi ongoing non-international armed conflict, they were not spilling over from the territory of a participant in a NIAC.

The better approach, and the only one that supports the application of IHL in a conflict between a State and an organized armed group outside the State’s territory (as distinct from support of another State within that State’s territory), is that IHL applies wherever hostilities with a nexus to the conflict take place. Whether a State may conduct such hostilities in the territory is determined, as explained above, by the law of self-defense. But so long as the State is lawfully operating in a location, IHL governs its actions. This is the U.S. position, and one I support.

The U.S. strike plainly complied with IHL’s “conduct of hostilities” rules. Since the groups used the facilities to transship arms, they qualified as military objectives by the “use” criterion (a civilian object used for military purposes). Those individuals who were killed or wounded were reportedly members of an organized armed group and therefore targetable on that basis. And although reports are still sketchy, there does not appear to have been significant collateral damage. Therefore, the strike conformed to the proportionality rule, which requires that attackers refrain from attacking when the expected collateral damage is likely to be excessive relative to the military advantage of conducting the operation. Finally, IHL demands that an attacker do everything feasible in the circumstances to minimize collateral damage. In this case, U.S. forces selected remote targets, conducted the strike in the early morning hours when civilians were unlikely to be present, and employed precision munitions; accordingly, the requirement to take “precautions in attack” was satisfied.


The law of self-defense is, as illustrated above, subject to numerous ambiguities. Whether the U.S. strike met with the requirements for acting in self-defense depends on one’s position on them. In my estimation, the operation complied fully with the most defensible interpretation of each of the conditions for self-defense. Moreover, the manner in which the United States conducted the strike serves as a model for applying IHL’s obligations and prohibitions, especially that requiring precautions in attack. The February 25 airstrikes were lawful.


Michael N. Schmitt is the G. Norman Lieber Distinguished Scholar at the United States Military Academy. He is also Professor of Public International Law at the University of Reading, Strauss Center Distinguished Scholar and Visiting Professor of Law at the University of Texas, and Charles H. Stockton Distinguished Scholar-in-Residence at the United States Naval War College.