In a recent essay in Just Security entitled “Iranian Gunboat Harassment and the Rules of Engagement,” Michael Schmitt and Durward Johnson explore important questions raised by President Trump’s recent instruction to destroy any Iranian gunboats that harass U.S. warships in the Persian Gulf. It is a thorough and careful analysis, and I agree with its conclusions – namely, that the harassing actions in question did not rise to the level of an imminent or actual armed attack on U.S. vessels, and thus could not justify a use of force in response, and that the president’s statements could in turn constitute an unlawful threat to use force in violation of Article 2(4) of the U.N. Charter. But the essay also raises some interesting and debatable questions that invite further discussion, and it makes assertions about the state of particular principles of international law that require some push-back. Let us begin with the latter.

The State of the Law: The Law of State Responsibility

The authors identify their purpose as assessing whether U.S. warships have a right under the body of international law that governs the resort to force by states (jus ad bellum), to use force in self-defense against Iranian gunboats engaged in harassment operations. At the very outset, in laying the foundation for the jus ad bellum analysis, the authors state that “as a matter of law, self-defense is a ‘circumstance precluding wrongfulness’ of a state’s use of force that would otherwise violate the prohibition found in Article 2(4) of the U.N. Charter…”

This is actually a statement about the operation of the law of state responsibility, not jus ad bellum, and it reflects a common misunderstanding of the law. Now, Article 21 of the International Law Commission’s Articles on Responsibility of States for International Wrongful Acts does indeed provide that “[t]he 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.” But, as Federica Paddeu has persuasively explained in her outstanding article on precisely this issue, Article 21 was intended only to excuse the collateral violation of other international legal obligations in the course of the lawful exercise of self-defense, not any violations of Article 2(4) of the Charter.

This is because Article 51 and Article 2(4) of the U.N. Charter operate together such that the lawful use of force in self-defense, in accordance with Article 51, is no violation of the prohibition on the use of force in Article 2(4). Thus, no additional legal excuse or justification is necessary. The lawful use of force in self-defense may, however, incidentally result in the collateral violation of other obligations — in relation to non-intervention or freedom of commerce and navigation, for instance — for which Article 51 of the Charter provides no defense. It is these violations that the circumstances precluding wrongfulness operate to excuse.

To be clear, the authors’ statement regarding the circumstances precluding wrongfulness in no way affects the outcome of their argument, but it is important that this common misunderstanding of the relationship between the jus ad bellum regime and the law of state responsibility not be perpetuated by repetition.

The State of the Law: Jus ad Bellum

In further laying the foundation for the jus ad bellum analysis, the authors also fairly acknowledge that the U.S. positions on the interpretation of certain key principles and concepts depart from those of the International Court of Justice (ICJ). Thus, for instance, they flag the minority position of the United States in treating an “armed attack” as any use of force that would violate Article 2(4) of the Charter, as opposed to the more widespread and established view that an armed attack is limited to “the most grave forms of the use of force,” as held by the ICJ in Military and Paramilitary Activities in and Against Nicaragua (Nicaragua v. United States of America).

Having acknowledged this difference, however, the authors go on to suggest that an attack by the military forces of one state against the warship of another would “certainly amount to an armed attack” under the law of self-defense according to either standard. In support of this they cite the Oil Platforms (Islamic Republic of Iran v. Unites States of America) case, in which the majority “refused to exclude ‘the possibility that the mining of a single military vessel might be sufficient to bring into play the inherent right of self-defense.’” But recognizing that an attack on a warship could, in some circumstances, constitute an armed attack, does not meant that any and all attacks on warships will cross the threshold.

There is a significant difference between the mining of the Samuel B. Roberts, the ship in question in Oil Platforms, and a single strafing of a ship with a .50 cal. machine gun mounted on the bow of a gunboat, just to take one hypothetical. The Samuel B. Roberts was actually incapacitated by the mine-strike, and almost sank — for a harrowingly detailed account see chapter 17 of The Twilight War. The harassment by Iranian gunboats at issue in recent events, in contrast, did not even involve any shots fired. But it will be recalled that even the mine attacks on tankers in the Persian Gulf in the summer of 2019, for which the United States accused Iran, did not seriously threaten to sink the mined ships. Just as mere frontier skirmishes do not amount to an armed attack or trigger the right to self-defense, so too some low-level attacks against a warship may not either. At some point along this spectrum, the difference between the American and international characterizations of the concept of “armed attack” is going to be material in determining whether a right of self-defense exists.

Similarly, in introducing the subject of anticipatory self-defense, the authors claim, “It is, indeed, an essentially universal view in international law that anticipatory self-defense is permitted, and the unsettled question is what exactly counts as imminent.” This is not an accurate statement. No one can read Tom Ruys’ careful and precise analysis of the doctrine of self-defense, supported by extensive evidence of both state practice and opinio juris, and conclude that anticipatory self-defense is “universally accepted.” Ruys’ analysis suggests that there has indeed been movement toward acceptance of the principle, but it is not even clear that it has yet become the majority view, let alone universal. Both he and Yoram Dinstein endorse a much more restrictive concept of “interceptive self-defense.” To be fair, many other voices, and even the U.N. Secretary General’s High Level Panel on Threats, Challenges and Change, A More Secure World: Our Shared Responsibility, suggest that at least a restrictive understanding of anticipatory self-defense is now accepted, but it is overreaching to say it is universal. The Non-Aligned Movement, in particular, continues to resist expansive interpretations of Article 51.

What is more, in then turning to the question of the content of the concept of imminence, the authors trot out the factors that the United States employs to determine imminence. But this conception of imminence, developed in large measure as part of the concept of “preventative self-defense” or the so-called “Bush Doctrine,” and carried over to the “unwilling or unable” doctrine, remains highly controversial (as I have analyzed in more detail elsewhere) and is far from established or accepted as part of existing law. In trying to objectively assess the legality of some act or claim, it does not do to rely on the interpretations of key concepts that are themselves the subject of great debate and uncertainty.

Conflating Rules of Engagement with International Law

This problem of taking the U.S. interpretation of the law as representing the actual state of the law is compounded by the authors’ focus on the U.S. Standing Rules of Engagement (SROE). Again, the authors’ stated purpose is to assess whether U.S. warships have a right under jus ad bellum to use force in self-defense against Iranian gunboats; but they then go on to say that they will, “in particular,” examine the U.S. SROE.

As interesting and important as an analysis of the SROE might be, it is important not to conflate them with the principles of the jus ad bellum. While rules of engagement, which are a matter of national policy, should be consistent with principles of both jus ad bellum and international humanitarian law (IHL), they may not be. They are frequently more restrictive than the law, but at times they may also depart from the law. And in the authors’ exploration of how the U.S. SROE purportedly implement principles of the jus ad bellum regime, they illustrate some significant departures.

The authors explain, for instance, how the SROE “translate ongoing and imminent armed attacks” into the “operationally understandable terms ‘hostile act’ and ‘hostile intent.’” A use of force in response to a manifestation of hostile intent is said to be synonymous with anticipatory self-defense in response to an imminent armed attack, but is defined as including a use of force against:

the threat of imminent use of force against the United States, US forces or other designated persons or property. It also includes the threat or use of force to preclude or impede the mission and/or duties of US forces, including the recovery of US personnel or vital USG property.

Even assuming the validity of anticipatory self-defense, an expansive interpretation of this SROE clause — particularly the phrase that justifies self-defense in response to a threat to use force to “preclude or impede the mission and/or duties of US forces”— would not comport with most understandings of jus ad bellum doctrine. The SROE clause would be consistent with the established international legal understanding only if it were very narrowly construed to justify a response to an armed attack directly against the armed forces of the United States — but in that case it would seem entirely redundant. Meanwhile, just this past week, the U.S. Navy issued a broadcast warning that any armed vessels coming within 100 meters of U.S. naval vessels in the Persian Gulf, may be “interpreted as a threat and subject to lawful defensive measures.”

My aim here is not to fully explore the daylight between the U.S. SROE and the jus ad bellum, but simply to suggest that if one is trying to explore the legality of some action under jus ad bellum, the SROE is not the right lens for doing so.

Questions Regarding Unit Defense and Jus ad Bellum

In applying the rules of engagement, the authors also suggest that the same principles apply to the right of self-defense whether it is exercised at the level of the state, the military unit, or even the individual. Fairly acknowledging that this is the subject of some debate, they endorse Yoram Dinstein’s view that there is “a quantitative difference but no qualitative difference between a single unit responding … and the entire military structure doing so.” This raises interesting questions, which require more detailed exploration than can be engaged in here, but let me sketch out some preliminary thoughts on the relationship among the three levels.

First, I agree with the authors that in general the principles of self-defense are similar at all three levels — that is, the victim of an armed attack may respond with a use of force, so long as it is necessary and proportionate. Assuming that anticipatory self-defense is valid, this includes a response to an imminent armed attack. But the principles at these three levels, while similar, are actually operating in different legal regimes, and the content of the concepts and principles of the doctrine differ somewhat across the legal regimes.

The authors appear to be limiting their discussion of the individual level to the individual member of a unit engaged in armed conflict, which partially eliminates any confusion regarding the application of law enforcement and IHL principles. Thus, the incident of an American border guard shooting and killing a Mexican boy in Mexico, which was recently before the Supreme Court, obviously did not implicate either jus ad bellum or IHL considerations, and is apparently not the kind of incident that the authors are referring to here. But even if we limit our focus to the relationship between the unit and the state level, it is not clear that the jus ad bellum principles apply to both.

The key question is whether any and all attacks on a unit constitute a use of force, or an armed attack, such that the act of unit self-defense is an operation of jus ad bellum. Beginning with the initial use of force itself, there is some debate over whether there is a threshold of gravity and intensity for a violent military action to constitute a “use of force” for purposes of Article 2(4) of the Charter. On this view, there is a spectrum, with the lowest levels of violent military action — such as targeted killing of a single individual or the interception of a single aircraft — falling below the threshold of “use of force,” through to the “use of force” prohibited by Article 2(4), all the way up to the gravest form of use of force that constitutes an “armed attack.” Tom Ruys reviewed this debate in a law review article, and concluded that the better and more established view is that there is no such gravity/intensity threshold for the use of force, but there are some well-known scholars, such as Olivier Corten, Robert Kolb, and Mary-Ellen O’Connell, on the other side of the debate.

Even if we accept this view that the lowest level of hostile and violent action toward a military unit would constitute a use of force in jus ad bellum terms, many such uses of force would certainly not rise to the level of armed attack as discussed above. Thus, here again the difference between the American position and the more widely held view of the doctrine of self-defense, will lead to different conclusions regarding the application of jus ad bellum to some unit-level action. If we accept the mainstream view that there is a higher threshold for armed attack, and thus for the exercise of self-defense, then some low-level strikes on military units would not justify an exercise of self-defense at the state level, but they would certainly justify the unit in responding with force in an act of unit self-defense under the rules of IHL. And, again, the content of the principles governing this exercise of self-defense will include IHL rules such as those regulating targeting operations, but will not include jus ad bellum rules operating at the state level. In this context the SROE discussed by the authors would apply, but as an implementation of IHL, not jus ad bellum.


The Schmitt and Johnson essay is to be commended for raising these last questions. Issues surrounding the relationship between jus ad bellum and IHL always deserve further exploration. How these two legal regimes relate to one another, and how the law operates at the seams between them, often gets too little attention. And the final conclusions that the authors reach regarding the proposed American actions’ compliance with established U.S. interpretations of legal and policy frameworks are obviously of extreme importance. But while there is some rhetorical power in demonstrating that such actions would even violate the American views of international law, one must be very clear about what one is doing. There is a risk of feeding into a common tendency of conflating the American minority positions on some aspects of international law with more widely accepted and established principles of law.

Image: The guided missile cruiser USS Vella Gulf fires a close-in weapons system during a live-fire exercise in the Arabian Sea, April 27, 2020. Photo by Navy Petty Officer 3rd Class Andrew Waters/U.S. Department of Defense.