The Rules of Engagement Are the Wrong Lexicon for Deterrence Signaling

Eleven Iranian-flagged vessels executed “dangerous and harassing approaches” directed at U.S. ships in the Arabian Sea on April 15. According to U.S. Central Command, the repeated harassment of six U.S. ships continued for an hour, despite U.S. use of deescalatory measures including auditory warnings and attempts to establish bridge-to-bridge communications. After coming within 10 yards of collision with the USCGC Maui (WPB-1304), the Iranian vessels finally answered radio queries and disengaged.

A week later, in apparent response to the incident, President Trump tweeted, “I have instructed the United States Navy to shoot down and destroy any and all Iranian gunboats if they harass our ships at sea.” Reportedly, the statement appeared to take Pentagon officials by surprise. Under the Standing Rules of Engagement (ROE), military forces are already authorized to use force in response to a hostile act or demonstrated hostile intent under their “inherent right to self-defense.” The president’s tweet appeared to expand this standard by ordering force in response to merely “harassing” behavior. The situation fizzled after the president later stated there was no need to change the ROE, while Iran dismissed the accusations of maritime harassment as a “fake Hollywood tale.”

The April 15 incident is simply the most recent in a much longer history of waxing and waning U.S.-Iran hostilities, which have sometimes challenged the definitional boundaries of armed conflict under international law. Other recent incidents include the U.S. strike that killed Iranian Maj. Gen. Qassem Soleimani and Iran’s retaliatory attack a few days later.

These flashpoints are often framed by policymakers and scholars in terms of legality or deterrence effectiveness, but rarely both. This is an oversight: The legal language used by the U.S. about such incidents often undercuts the deterrent value of U.S. statements. U.S. government officials often rely on ambiguous references to “self-defense” or employ ROE terminology to explain the legal bases for the use of force. This creates confusion about America’s view of the law and dilutes the power of U.S. deterrence signaling—and, what’s more, limits U.S. responses to aggressive actions. The better approach is to use the lexicon of jus ad bellum, rather than ROE, in public messaging.

Incoherent U.S. Justifications for the Use of Force

Effective deterrence signaling depends on the existence of credible threats. For a threat to be credible, the threatening state must possess the capacity to inflict substantial costs against the target state and convince the target state that it is resolved to carry out its threats. Key to this process is the timely and transparent communication of sound legal bases to use force, which—if the threatening state is successful—ideally inspires domestic and international support. If the target state believes the threatening state’s use of force is likely to be viewed as lawful and justified by the international community, the target state will likely view the threat as more credible.

Vague or confusing messaging that blurs the nature of the threatened action undermines the adversary’s perception of credibility. This invites aggressive action, where the “motivated aggressor” can easily convince itself that the threatened response was either not a threat at all, or a mere bluff. Two brief examples highlight this point. First, during the rising Cold War tensions in Korea in the late 1940s, Secretary of State Dean Acheson implied “that the Korea Peninsula lay outside the all-important ‘defense perimeter.’" This obviously did not deter the aggressive military ambitions of North Korea in 1950. Second, in 1990, when the U.S. became concerned with the Iraqi military buildup on the border with Kuwait, instead of sending a clear message to Saddam Hussein, the “State Department prepared equivocal statements for the Administration about American commitments to Kuwait.” This too failed to deter Iraq from invading Kuwait.

Unfortunately, U.S. messaging on the use of force continues to confuse international and domestic audiences alike, in such a way as to undercut U.S. credibility and embolden its adversaries. Two recent examples highlight the U.S. tendency to shift confusingly between legal paradigms, sometimes justifying the use of force using ROE or vague self-defense terminology, other times conflating peacetime self-defense rules with the law of armed conflict, and occasionally sprinkling in concepts from the jus ad bellum.

The first example is from June 2017, when the United States shot down a Syrian Air Force Su-22 jet in Syria. The United States explained that the strike was justified under the “legal” authority of “collective self-defense” pursuant to the ROE in protection of U.S. counter-ISIS partners, the Syrian Defense Forces (SDF). This claim drew significant questions under both international law and domestic law, as it inartfully blended the ROE concept of collective self-defense with the United States’s jus ad bellum right of self-defense under Article 51 of the U.N. Charter. These frameworks are discussed in detail below, demonstrating why reliance on the ROE lexicon to explain the legal basis for military actions often breeds confusion.

The second example came in January 2020, when the United States struggled to offer a consistent legal basis for the strike on Soleimani. A full discussion of the evolution in legal arguments is beyond this article’s scope, but it is important to highlight a few points (see here and here for an in-depth legal analysis and commentary on the strike itself). The U.S. justification initially focused on prevention of an imminent attack that Soleimani was planning. Use of the word “imminent” combined with references to future “attacks” made it appear the United States was relying on anticipatory self-defense under jus ad bellum.

But this justification later shifted, with officials focusing instead on a past “escalating series of armed attacks by Iran and by Iran-supported militias in the Middle East” targeting the United States. Abandoning the initial anticipatory self-defense argument, the new U.S. justification seemed to rely on the existence of an ongoing armed conflict, that is, a jus in bello paradigm. In this understanding, Soleimani was a lawful target under the law of armed conflict. Inspiring further confusion, Trump communicated to both domestic and international audiences that the strike was intended to “deter” Iran from continuing to threaten the United States. These statements essentially reframed the legal relationship between the United States and Iran back to a peacetime, or prearmed conflict, legal paradigm, despite the availability of other rationale. Thus, any implied threat of continued U.S. military action made to deter additional Iranian aggression was undercut by placing the United States back into peacetime rules for using force. This shifted escalation control back to Iran—as shown by Iran’s naval harassment and failed test fire of an anti-ship missile technology.

The April 2020 incident in the Arabian Sea presented the Department of Defense with a fresh opportunity to clarify its legal basis for using force in response to Iranian naval harassment. Yet the clarity was limited: The Defense Department’s ambiguous justification incorporated ROE concepts of hostile act and the inherent right to self-defense. Effectively, this boxed the Pentagon into a reactionary peacetime paradigm in which Iran maintained escalation dominance and initiative. Under the ROE framework, U.S. attacks on the Iranian vessels would be unjustified, raising doubt internationally about U.S. willingness to defend itself under jus ad bellum. Almost one month later, the United States was still struggling to offer a clear legal basis for its use of force against Iran. The military announced a new posture “designed to enhance safety, minimize ambiguity and reduce the risk of miscalculation” vis-a-vis foreign vessels approaching U.S. war ships, warning that “lawful defensive measures” may be taken.

If the United States had instead initially threatened to use force under jus ad bellum, Iran would have been forced to consider whether its continued actions were worth risking the devastating consequences of initiating an armed conflict, or at least a limited but significant kinetic response under jus ad bellum self-defense. The president’s new position to “shoot down and destroy any and all Iranian gunboats,” presumably under jus in bello rules, is an admittedly more challenging position for the United States to defend after abandoning this legal basis regarding the Soleimani strike. Signaling its willingness to target and attack Iranian boats under a jus in bello paradigm was a high-cost commitment for the United States to make with potentially significant deterrent effects. However, the United States subsequently dulled the deterrence value of this commitment by adopting the legal paradigm that would render such actions unlawful.

If the United States continues to use self-defense language to justify the use of force or threaten aggressive actors, officials must select and consistently rely upon the legal paradigm and lexicon that will best support its deterrence strategy. As the following analysis of self-defense variants will show, the best legal paradigm to use is the jus ad bellum.

Self-Defense Across the Use-of-Force Paradigms

Self-defense exists in every use-of-force legal paradigm, but in different forms and subject to distinct rules. Each type entails different consequences for the target state—in this case, Iran—with correspondingly varying prospects for deterrence. The challenge lies in the fact that the different paradigms share similar, if not identical, terms of art—like necessity, proportionality, imminence, self-defense and others—with distinct, paradigm-specific meanings. When the United States uses general self-defense terminology without any more detail, therefore, it does not precisely identify an operative legal paradigm and cannot convey a specific consequence to the targeted adversary.

The importance of communicating using clear legal language is paramount for U.S. leaders—both domestically, in the service of a responsible democratic government that is accountable to its citizens, and also for the international audience. Both sides to a conflict usually try to justify their actions as legitimate acts of self-defense in response to the adversary’s illegal aggression. In selecting whether to rely specifically on self-defense under the ROE or the jus ad bellum, the United States must carefully consider which paradigm best serves its deterrence efforts while informing its citizens. Doing so ensures both public domestic and international support for our contemplated or threatened military actions. This, in turn, increases the perceived willingness by the United States to actually use force and enhances the deterrent effect of those communications.

Before turning to self-defense exercised by a state or its agents, the concept of self-defense in human rights and criminal law deserves brief mention. International human rights law (IHRL) provides the basis for individuals to use force in personal self-defense against illegal acts of violence. As a criminal law concept, self-defense is usually codified as an affirmative defense and requires a threat of immediate death or grievous bodily harm. Statutes may vary, but the immediacy requirement for personal self-defense under criminal law is more restrictive than the “imminence” requirement appearing in other types of self-defense. For example, service members retain the personal right to self-defense under the Uniform Code of Military Justice in criminal matters but may also exercise jus ad bellum self-defense as an individual agent of the state and in accordance with the lower imminence standard. Unhelpfully, the term “self-defense” is often used in the context of an ROE authority during ongoing armed conflict, but the actual law-of-armed-conflict contours of self-defense are exceedingly narrow and all but meaningless to use as a legal justification for state-level uses of force. Adding to the confusion, some criminal jurisdictions use the word “imminent” in their self-defense statutes instead of “immediate.” And to make matters worse, some organizations and partner forces require the criminal law immediacy standard for individual self-defense exercised by state agents. (For a full discussion of the interplay of soldier self-defense in various contexts, see Col. Randall Bagwell’s post here.)

Self-Defense in the Rules of Engagement

International human rights law also governs a state’s uses of force wherein its agents—either law enforcement officers or military personnel—may intentionally use force on its behalf during peacetime and against nonbelligerents during armed conflict. The law of armed conflict displaces IRHL as lex specialis to govern the use of force between belligerents during an armed conflict, but IRHL self-defense does not disappear with respect to threats unrelated to enemy forces.

Many entities—including states, international organizations such as NATO and even academic centers—produce rules of engagement as guidelines for military forces during peacetime and armed conflict. Rules of engagement vary widely and are best described as chimeras of legal policy mixing jus ad bellum, jus in bello, IHRL and domestic law along with military doctrine and strategic policies. They are designed for commanders to manage military forces and capabilities during combat and noncombat operations. Each state or organization that produces rules of engagement has its own strategic and political priorities, and varying treaty obligations—hence the resulting disparities in standards and operative terminology.

These disparities can create confusion in strategic messaging: The signaling state, targeted adversary and international community lack a common self-defense lexicon under their respective ROE paradigms. Even when a threat cites a specific set of rules under the ROE, classification restrictions and state-specific terminology inhibit universal comprehension of the precise circumstances that trigger a right to self-defense—not to mention the likely scope and magnitude of the defending state’s response. This resulting ambiguity may invite continued aggressive activities and encourage use of political, economic, informational and military capabilities in the gray zone between war and peace, depriving the victim state of traditional jus ad bellum and state responsibility legal tools to respond to such aggression.

Unless the United States is operating with partner forces or as part of a coalition, U.S. statements about “rules of engagement” refer to the U.S. military’s own ROE, which outlines two bases for the use of force: self-defense and mission accomplishment. Pertaining to self-defense, the ROE authorizes national, unit, individual and collective varieties, each potentially subject to various levels of approval and restrictions. The ROE operationalizes and defines self-defense terms of art, such as “hostile intent” and “hostile act.” Unit commanders always retain the “inherent” right to use force in response to a “hostile act or demonstrated hostile intent” but may also limit it for individuals in their units as appropriate. (As Bagwell notes, commanders may limit “individual’” self-defense under the ROE because it is exercised on behalf of the state and distinct from individual self-defense in criminal law.) Self-defense under the ROE is reactive and requires an “imminent” hostile act or demonstrated hostile intent. But unlike under the criminal law standard, imminence under the ROE “does not necessarily mean immediate or instantaneous” and will be evaluated “based on all of the facts and circumstances known to U.S. forces at the time.”

Taken together, the terms and standards in the U.S. ROE create a broader, more permissive right to self-defense than that of organizations like NATO and many U.S. partners, whose rules of engagement more closely mirror criminal law requirements.But self-defense under the U.S. ROE is not unlimited. Whenever feasible, forces committing hostile intent or acts must be warned and given an opportunity to cease their conduct. If the threat disengages, the event is over and the responding force may not use force in retaliation or engage in pursuit. (An exception exists for a coastal state’s pursuit of criminals who flee into international waters, but this would not apply to U.S. pursuit of Iranian vessels in the Arabian Sea.) The use of force in self-defense must also comply with the principles of necessity and proportionality, which seek to deescalate the situation quickly by significantly limiting the kind and duration of force used. “Necessity” under the ROE permits force in self-defense only as a last resort, when no other option exists to stop the hostile actor. “Proportionality” further limits force to the minimum required to neutralize the threat.

Notably, “necessity” and “proportionality” under the ROE have different meanings than the same terms do in jus in bello and jus ad bellum (and the law of state responsibility). This creates undesirable ambiguity when these terms are used as stand-alone terms in strategic messaging, especially when it is unclear whether a state of armed conflict exists.

Finally, and importantly, the ROE derives from U.S. domestic authority only, rather than international law. While the president’s constitutional authority to threaten force is clear from a domestic legal perspective, its contours do not align neatly with the rules of engagement of other states and organizations. In other words, not only does the ROE employ a confounding U.S.-specific lexicon that is difficult for other states to understand, but its standards do not always perfectly reflect international law. A legal basis rooted exclusively in domestic authority—and a more permissive authority than that appearing in other systems—is a poor source of international legitimacy.

Self-Defense During Armed Conflict

The jus in bello is a separate paradigm that largely displaces IHRL and jus ad bellum as lex specialis with respect to the use of force between belligerents. Under the law of armed conflict, also referred to as jus in bello, a soldier need not justify the use of force against her enemy in self-defense. She may engage her enemy solely based on his status as a member of the enemy force, regardless of whether he presents an immediate or imminent threat. The ROE operates both during peacetime and armed conflict—but during armed conflict, it expands beyond self-defense to include uses of force required for “mission accomplishment,” authorizing the status-based engagement of enemy forces.

Jus in bello uses of force are distinct from all varieties of self-defense and serve an entirely different purpose. Status-based rules are designed to force the enemy into submission and end armed conflicts as quickly as possible. Unlike self-defense under the ROE, deescalation is not the goal, and the use of force need not be a “last resort.” While not the chief focus of this article, jus in bello deserves attention because its principles are easily conflated with self-defense principles that are similar in name only. For example, the jus in bello principle of “necessity” is expansive and permits any sort of force unless expressly prohibited. It does not demand a “least restrictive means” to bring about the enemy’s submission and is therefore not self-limiting like its self-defense counterpart. Warnings that provide a hostile actor with an opportunity to disengage are accordingly not required. Jus in bello “proportionality” relates not to some measure of permissible force that may be used against the enemy but, rather, to the acceptable degree of collateral damage to civilians and civilian objects. Unlike self-defense in IHRL—and, as we will explore, in the jus ad bellum—there is no requirement to engage the enemy with the minimum amount of force likely to achieve the desired result.

Lastly, as it relates directly to self-defense in an armed conflict, instances of bona fide law-of-war self-defense are quite limited. Jus in bello status-based targeting allows deadly force as a first resort against the enemy—vastly different from the authority required to use lethal force outside of armed conflict, which otherwise requires a IHRL justification. A review of the Geneva Conventions (GC) and their Additional Protocols (API), the commentaries on them, and the Hague Regulations—the main corpus of modern jus in bello—reveals just seven articles related to self-defense: GC I Article 22; GC II Article 35; API, Articles 13, 65, and 67(1)(d). Prison guards also have some authority to use force against detainees, but this is categorically different from other instances of self-defense. Arguably, Hague IV, Articles 23(c) and 40 relate to a limited law-of-war right to self-defense as well. What is common in all these concepts of law-of-war self-defense is that the “right” is limited to those individuals who are not entitled to take actions harmful to the enemy and have a law-of-war protection, and these individuals can use self-defense only to defend against an illegal attack upon them. Properly marked medics who are protecting themselves from unlawful attack are the classical example of this narrow law-of-war right to self-defense. As mentioned up front, soldiers retain their IHRL-based right of personal self-defense during periods of armed conflict but only as it relates to illegal acts of violence against them with no nexus to the armed conflict.

Because the ROE incorporates rules related to jus ad bellum and IHRL self-defense on the one hand, and jus in bello status-based targeting on the other, the respective principles of jus in bello and jus ad bellum are often confused during times of armed conflict. Keeping the rules for self-defense separate from the jus in bello is essential for soldiers on the ground, who must decisively shift between paradigms to engage different types of threats. It can be particularly difficult for soldiers fighting in a counterinsurgency to keep the paradigms separate, as determining whether a hostile actor is a member of a declared hostile force may be challenging. Situations can become especially blurry when civilians take a direct part in hostilities and when non-state armed groups become involved.

The legal complexity of “soldier self-defense” results in part from this simultaneous, coapplication of self-defense principles with the jus in bello during armed conflict. While the ROE serves as guiding policy for the use of force under both paradigms, shared terminology is susceptible to blending at all levels, impairing practical application on the ground and strategic messaging alike. The fact that other states and organizations have their own rules of engagement—which may or may not align with the U.S. ROE—only increases this complexity.

Relying on the ROE to explain the legal basis of U.S. action thus creates significant problems and ambiguities for lawyers and leaders across the spectrum of operations from court cases to armed conflict, and certainly for the United States’s deterrence messaging.

Self-Defense in the Jus Ad Bellum

The third flavor of self-defense germane to deterrence signaling is the jus ad bellum, which is the body of law governing when and how states may resort to force. Customary international law and the U.N. Charter’s Article 2(4) generally prohibit states from using force unless an exception applies. One such exception is if force is authorized by the U.N. Security Council. The other exception is enshrined in Article 51 of the U.N. Charter, which permits state-level individual and collective self-defense “if an armed attack occurs.” Although Article 51 excludes mention of anticipatory self-defense, most scholars and practitioners agree that states may use force as a preventive measure before an armed attack occurs under certain circumstances. This view has been accepted, albeit with differing contours, since the 1800s and articulated in the Caroline affair.

The definition of “armed attack” is the subject of continuous debate. It means something less than aggression (a broader term that appears elsewhere in the Charter) and is inherently entangled with the concept of uses of force. It does not require a specific type of weapon, and may use unsophisticated means to achieve its effect. For a state to respond before the armed attack actually occurs, it must be “imminent.” Unlike the same term in the ROE or the immediacy requirement under IHRL, both of which are explicitly defined in statute, the meaning of imminence under jus ad bellum has never been authoritatively determined, and any coherent and meaningful insight of the concept has been muddled by the International Court of Justice.

Finally, whether a provocation constitutes an imminent armed attack depends on the perception of the would-be victim state. The subjectivity of this determination, along with the convenient obscurity of the jus ad bellum imminence standard, lends a defending state great flexibility in the formulation of its argument for resorting to force, the legality of which will be judged at a later time.

“Imminent” is not the only term that the jus ad bellum shares with other paradigms. For example, collective self-defense is a term of art under the ROE, but it has a very different meaning under jus ad bellum. This fact was the source of much confusion regarding the U.S. explanation for the 2017 shoot-down of the Syrian Air Force Su-22. (As a separate but important issue, there are divergent views on collective self-defense as a basis for the use of force, both under the ROE and the jus ad bellum.) Principles of “necessity” and “proportionality” appear here, too. Closely tracking its IHRL definition, “necessity” under jus ad bellum permits a state to use force in self-defense only as a last resort. In other words, the state must confirm that resolution of the conflict through amicable means is not possible.

A final point on jus ad bellum necessity and proportionality is critical to deterrence strategy and strategic messaging. With similar definitions, one might expect these principles to operate similarly in practice under IHRL and jus ad bellum paradigms. However, this is not the case. As Yoram Dinstein notes, jus ad bellum necessity and proportionality best restrain “on-the-spot” state reactions to isolated armed attacks. Isolated attacks mirror incidents effectively addressed by the ROE, which requires warning hostile actors when feasible and restricts the magnitude of the responding force. But if an armed attack constitutes one in a series (as in the present U.S.-Iran state of affairs), then jus ad bellum necessity and proportionality are less likely to restrain a swift and devastating state response. The fact that other measures were unsuccessful in preventing past attacks in the series may satisfy the necessity requirement. Proportionality, as Dinstein asserts, is a useless measuring stick for force in a situation where a “war of self-defense” results. In a “war of self-defense”, or de jure armed conflict, the jus in bello supplants jus ad bellum and generally permits the destruction of all enemy forces in order to end the conflict quickly and efficiently.

Therefore, the threat of triggering jus in bello targeting rules is a severe consequence for an aggressive, yet militarily weaker actor like Iran vis-a-vis the United States. Credible warnings that the aggressor’s continued actions will cross the low threshold to trigger an armed conflict signal a costly consequence to Iran: If the U.S. changes the paradigm from allowing a limited jus ad bellum self-defense response to allowing more permissive jus in bello attacks, Iran risks sustained U.S. military action from which it would be unable to defend itself. If Iran is convinced that its harassing and aggressive behavior may lead the U.S. to change the paradigm in a way that strongly disfavors Iran, its leaders may curb its harassing and aggressive activities.

Selecting the Right Legal Lexicon Enhances Deterrence Signaling

Because legal authority to use force stems from jus ad bellum, jus in bello and IHRL, the relevant law can be complicated to identify and apply, especially during armed conflict with multiple paradigms operating. Even when the source of authority is clear, national policies are inconsistent, and shared terminology obscures a common understanding of paradigm-specific concepts. Tactical and operational practitioners often experience confusion—and this is magnified at the strategic level, where leaders are far removed from the battlefield yet are responsible for official state communications designed to influence adversaries and engender support from allies, neither of whom may precisely grasp U.S-specific legal concepts.

Self-defense as it appears in domestic authority gets even more complex with the introduction of international law concepts and terminology from the jus ad bellum, jus in bello, and IHRL. This aggravates existing challenges in strategic messaging as a singular communication invites innumerable interpretations. Divergent interpretations undermine clarity and credibility, both of which are essential to communicating the resolve to act.

When a state threatens to use force against a would-be enemy in self-defense, its word choices implicitly communicate its belief as to whether an armed conflict exists, or may be triggered. If an assessment is expressed in ROE terms, the responding state is announcing that a particular provocation is finite and isolated, presumably far from the threshold of armed conflict. Using the concept of self-defense from jus in bello, by contrast, is useless for deterrence and legal messaging: The concept is limited and nuanced, and it presupposes the existence of an armed conflict. Alternatively, using jus ad bellum language may imply that states are at the brink of an armed conflict, a potentially costly position for the weaker and aggressive state.

But simple references to “self-defense” or the use of shared terms in absence of context are useless to indicate an operative paradigm. They fail to clarify the consequences for an adversary’s continued action. This means that the adversary is unable to perform a “deterrence calculus” to decide whether a certain course of action is worth risking a particular result and is thus less likely to modify its behavior.

To offer a final illustration, the use of the word “imminence” in reference to January’s Soleimani strike proved confounding: It was unclear whether the United States was invoking ROE language or declaring that the attacks planned by the general triggered a U.S. right to self-defense under Article 51. Since “imminence” means different things in the ROE versus jus ad bellum, Iran was ill equipped to anticipate when its behavior was likely to induce a U.S. response. Further, because the quality and magnitude of a response in self-defense is subject to different restrictions in each paradigm, Iran was similarly unable to weigh and calculate what consequences would likely result from its actions. For strategic messaging to serve as an effective deterrence tool, it must provide enough information for the adversary to calculate risk and determine that the consequences are too costly.

Turning to the April 15 incident in the Arabian Sea, the United States again failed to articulate a strong, paradigm-specific legal basis to use force in self-defense and failed to send a strong deterrence message to Iran. As presidential tweets seemed to eventually settle on the ROE as the “legal” basis for its response, it nonetheless took multiple attempts by senior officials over several weeks to clarify and address this ambiguity, only to then rely on the most problematic source of authority. The ROE borrows terms of art from several legal paradigms, mixes them together, and gives them U.S.-specific meanings. Self-defense under the ROE is inherently reactive and implies a self-limiting array of responses that lock the United States into a peacetime paradigm, surrendering escalation control to Iran. Iran’s recent failed demonstration of its anti-ship missile capabilities is undoubtedly an attempted show of naval capability to strengthen its own deterrence posture against the United States. While the demonstration failed, the timing of its execution suggests that Iran will continue to exercise the escalation control it is afforded under the ROE and does not perceive U.S. threats of attack to be credible.

If the United States had further developed its messaging highlighting Iran’s “series of escalating attacks,” including the Jan. 8 retaliatory missile attack, it would have strengthened the legal narrative that the two states were approaching a state of armed conflict. The United States could then have signaled its belief that Iran’s actions were going to trigger an international armed conflict, moving the United States out of the limited jus ad bellum self-defense paradigm for using force and into the permissive jus in bello paradigm. An armed conflict would be devastating for Iran economically, militarily and reputationally. Presumably, knowing these consequences, and believing the U.S. was committed to act, Iran would have responded by curbing its behavior to avoid this consequence. The United States would thus have achieved actual deterrent effects from its threats.

Explicit use of the jus ad bellum lexicon in this manner would improve clarity of U.S. threats and increase the legitimacy of U.S. strategic messaging while restoring escalation control to the United States. Setting a clear and legitimate legal narrative for both domestic and international audiences is an important pillar to strengthen the United States’s deterrent posture and more effectively curtail Iranian aggression.