Armed Conflict Foreign Relations & International Law

The U.S. Should Communicate in the Jus ad Bellum Lexicon to Strengthen Its Deterrence Posturing

Matthew J. Aiesi, Amanda L. Minikus
Thursday, June 25, 2020, 8:01 AM

Strategic messages that incorporate credible threats under jus ad bellum are often the best option to enhance deterrence signaling.

A boat flying the Iranian flag crosses the bow of a U.S. ship in the Persian Gulf, April 15, 2020. (Source: Official U.S. Navy photo)

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Editor’s Note: In a previous post related to this topic, the authors argue the U.S.’s reliance on the Rules of Engagement has failed to provide strong legal bases for its threats to use force in self-defense.

Deterrence has always been a central pillar of American foreign policy and national security objectives. The United States has traditionally rested on its nuclear and conventional arsenal coupled with economic power to enable its deterrent posture. Competition between states and non-state actors has expanded into new domains like space, the sea bed and the electromagnetic spectrum. The law, too, in its regulation of these domains has increasingly become a contested space. In this environment, the effectiveness of U.S. efforts to deter its adversaries is declining. While the United States has largely unrivaled military power compared to its state competitors, it often struggles to communicate the legal basis for its use of force. It is in this legal and policy context that we argue for deliberate use of the jus ad bellum lexicon and paradigm by policymakers to create and communicate more effective deterrence policies and take decisive actions that comply with domestic and international law. Using the recent escalating tensions between the U.S. and Iran as an example, this post highlights how the jus ad bellum better supports deterrence signaling.

To the detriment of its deterrence goals, the United States has consistently failed to communicate clear deterrent signals to its adversaries. The U.S. has incoherently shifted between legal paradigms in its threats to use force in response to recent Iranian aggression, often using general self-defense terminology or U.S. Rules of Engagement (ROE) language in its messages. Despite shared terminology, ROE “self-defense” has different standards and consequences for a target than self-defense under jus ad bellum, jus in bello or international human rights law. (For an in-depth overview of self-defense across the use-of-force paradigms, see our first post on this topic.) With a uniquely U.S.-centric meaning, ROE “self-defense does not translate well to international audiences and thus does not provide clear deterrent signaling, even when unambiguously identified as the source of authority. This is amplified by the fact that the key mission-specific rules are classified and cannot be used in threats to establish a strong deterrent posture. More importantly, ROE self-defense is inherently reactive and limited: It requires some actor to first demonstrate hostile intent or commit a hostile action, and only then may the U.S. actor, circumstances permitting, respond with force. That response must attempt to deescalate the situation—pursuit and capture are generally not authorized if the hostile actor disengages. Thus, not only are its complex standards and confusing terms U.S.-specific, use of the ROE in strategic messaging implies a self-limiting response and locks the United States into a peacetime paradigm that surrenders escalation control to the adversary. In contrast, the jus ad bellum has none of these defects.

For a threat to be effective, it must be clear, credible, legitimate and backed by the capacity to act. Clarity ensures an adversary’s precise comprehension of the threat. This comprehension, including an understanding of the legal paradigm upon which the deterring state acts, is required so that the adversary can evaluate the cost of a promised consequence. Credibility relates to the adversary’s belief that the threatening party has both the capacity and the willingness to act as promised. Inextricably tied to legitimacy, credibility is rooted in sound legal bases that foster international and domestic approval for uses of military force. It is only when these elements coalesce with the other pillars of national power, forcing the adversary to weigh the benefits of their actions against the threatened costs, that effective deterrence can be achieved.

Deterring malign Iranian action and influence is explicitly central to the United States’s National Security Strategy. Doing so, in part, involves issuing credible threats to employ the instruments of national power—including the use of military force—to discourage aggressive Iranian policies and behavior. But due to past reliance on the ROE and incoherent shifts between legal lexicons, the U.S. has consistently failed to convey credible threats and send deterrent signals to its adversaries. Most recently, the Department of Defense again struggled to articulate the condition in which U.S. warships may legally use force when they (and others) are threatened at sea. In April, Iranian vessels harassed U.S. ships in the Arabian Sea, and the president tweeted threats to use force in response. Attempting to clarify the presidential tweets, the Defense Department offered an ambiguous explanation that referenced ROE concepts of hostile act and the inherent right to self-defense. (For an in-depth analysis of the application of the U.S. ROE to the Iranian speedboat provocations, see this article.) Problematically, invocation of the peacetime rules for the use of force signals to the world that the U.S. would not be justified in attacking Iranian vessels for their harassing activities. It correspondingly weakens the U.S. deterrence posture by raising doubt about U.S. willingness to make the high-cost commitment to use force under jus ad bellum. The U.S. should instead frame its threat signals in terms of the jus ad bellum in order to craft strategic messaging in a manner that enhances its deterrent posture.

Explicit and exclusive use of the jus ad bellum lexicon would reduce ambiguity in U.S. communications to its adversaries, including Iran, thereby increasing U.S. legitimacy and restoring U.S. control over when and how hostilities escalate into an armed conflict. These benefits are consistent with modern deterrence theory and accordingly provide the best chance for the U.S. to actually influence Iran’s behavior. For three reasons we will discuss below—improved clarity, increased legitimacy and escalation control—the United States should explicitly incorporate jus ad bellum terminology and/or outright reference to the paradigm by name in its strategic messaging.

Deterrence Theory and Iran

The United States acknowledges that deterrence in today’s world is significantly more complex than during the Cold War. However, the fundamentals of deterrence theory as developed during the Cold War are still applicable today. In January, Secretary of State Mike Pompeo explained the concept of deterrence as follows:

In strategic terms, deterrence simply means persuading the other party that the costs of a specific behavior exceed its benefits. It requires credibility; indeed, it depends on it. Your adversary must understand not only do you have the capacity to impose costs but that you are, in fact, willing to do so.

Implicit in Pompeo’s statement is that effective deterrence requires clearly communicating (or signaling) to the target both the capacity to carry out the military action and the willingness to do so. This communication can be explicit or implicit. For instance, a state may threaten to use force explicitly through official statements or implicitly through the positioning of its military forces near the adversary. “Credibility” refers to the degree to which the adversary believes the threatening state is truly willing to use force. Lastly, “capacity” refers to the state’s actual ability to carry out the threat. While the United States enjoys an unquestionable advantage over Iran across all pillars of traditional national power—and therefore America’s capacity to decisively engage the Iranian Navy is not in doubt—the U.S.’s perceived willingness to use force is dubious due to the ambiguity of its threats. This stems in part from the vague and shifting legal bases for the threatened use of force.

As Pompeo describes, an effective threat must make the target decide that the benefits of pursuing its present course of action are not worth the costs if the threat is carried out. Linking the derrent message to a target state’s vital interests is thus critical in crafting an effective threat. In the case of Iran, the Defense Intelligence Agency outlined Tehran’s vital interests in a 2019 assessment of Iran’s military power, stating:

Iran has consistently demonstrated a preference for using partners, proxies, and covert campaigns to intervene in regional affairs because of limitations in its conventional military capabilities and a desire to maintain plausible deniability, thereby attempting to minimize the risk of escalation with its adversaries.

This conclusion is also supported by a Rand Corp. study from 2019 that found:

the broad consensus within the [government of Iran] is that Iran should have a strong military and the capabilities to deter enemies and raise the costs of conflict. Fundamentally, Iran’s defense posture is based on deterrence.

Put another way, Iran’s aggressive actions at sea are an attempt to project strength, signaling to its adversaries a credible and high-cost response to any state that would challenge Iran. Tehran aims to deter such actors from attacking Iran, without actually escalating into an open war.

Projecting strength while avoiding open conflict is a prime national objective for Iran. Therefore, Iran must maintain control over if and when a situation escalates. By crafting threats to Iran with the jus ad bellum lexicon, the United States can convincingly signal its commitment to use force against Iran in open conflict, depriving Tehran of escalation control. While simultaneously conveying the legality and legitimacy of a threatened use of force, the jus ad bellum paradigm also produces beneficial strategic ambiguity (discussed in detail below) as to the scope of a U.S. response to Iranian aggression. However, it avoids the detrimental ambiguity as to whether a response will actually occur that is produced by other paradigms, and is thus more likely to achieve the desired deterrent effect.

While there is some disagreement over what exactly constitutes an unlawful threat to use force, it is well established that “if [the threat] is to be lawful, the declared readiness of a State to use force must be a use of force that is in conformity with the Charter.” A threat of force that is nested under a state’s legitimate right to act in self-defense is thus a lawful threat and not aggressive. If the United States is able to craft an argument for the use of force under jus ad bellum that reasonably complies with the definitions of necessity and proportionality under that paradigm, the threat will garner greater international support than one that relies exclusively on domestic authorities, such as the ROE. The former will be perceived to reflect greater international legitimacy and will likely result in a correspondingly greater deterrent effect.

The Jus Ad Bellum Lexicon Enhances the Credibility of Strategic Deterrence Signaling

It is in Iran’s national interest to avoid an open and conventional armed conflict with the United States. The U.S. can exploit that interest to achieve a desired deterrent effect. Leveraging U.S. military strength and establishing upfront the legitimacy of its use aligns directly with Iran’s interest in avoiding an open conflict and will likely be a consideration in Iran’s policy formulation. The jus ad bellum paradigm and lexicon provides the perfect tool for the United States to credibly and legally threaten Iran in this manner. As we demonstrate below, consistently employing this lexicon will improve clarity of U.S. messaging by using a shared language, which in turn establishes the actual and perceived legitimacy of our military actions, and ultimately strengthens the deterrent posture of the United States by establishing escalation control. It provides the United States with a basis to consistently signal that Iran’s “series of escalating attacks” that constitute “uses of force” or “armed attacks” are on the razor’s edge of crossing from a limited tit-for-tat responseunder the jus ad bellum self-defense framework into a legal armed conflict wherein the limiting jus ad bellum principles of imminence, necessity and proportionality are replaced by the jus in bello. An armed conflict would be devastating for Iran, as under the jus in bello Iranian naval forces would be subject to attack under the law of armed conflict regardless of their actions or demonstrated intent. Facing a threat that implicated such grave consequences, Iran would be more likely to change its behavior to the benefit of the United States.

Improved Clarity

Jus ad bellum provides an internationally recognized use-of-force paradigm and a legal lexicon that constitutes a nearly universal language. Consequently, its use in strategic messaging can more clearly communicate a precise consequence that an adversary desires to avoid—and thus achieve effective deterrents. States are practiced in using the familiar jus ad bellum lexicon, and its concepts are mostly understood by members of the international community, enabling effective communication across numerous international audiences. The other paradigms that govern the use of force, such as international human rights, jus in bello or the ROE, are vague and confusing at best and counterproductive at worst. Universally understood—and broad—definitions of jus ad bellum terms provide the United States with flexibility in formulating its legal basis to use force without sacrificing credibility. Several jus ad bellum terms warrant brief discussion.

First, a state’s use of force in self-defense must be both “necessary and proportionate.” These terms have entirely different meanings than their variants under jus in bello (and under the U.S. ROE, as discussed previously). “Necessity” predicates any use of force in self-defense under jus ad bellum and means that no reasonable alternative means of redress are available to the victim state—including diplomatic efforts, which must be exhausted or deemed fruitless in stopping an armed attack. This may seem like a high bar, but it is not difficult to imagine draft talking points for key government officials meeting this test by running the gauntlet on Iran’s illegal activities, state sponsorship of terrorism, destabilizing regional intervention, attacks against U.S. drones and responsibility for the deaths of hundreds of Americans in Iraq. A consistent and coordinated rollout of this narrative in formal diplomatic channels, informal channels and public discussion would decidedly portray Iran as a pariah state actor whose aggressive actions are dangerously close to triggering a U.S. response rightfully justified by self-defense. “Proportionality” is more nebulous but generally refers to limits on the permissible force that a defending state may use to achieve an end to ongoing and future armed attacks. Importantly, this is quite a different standard than the same term under the jus in bello and the ROE, where “proportionality” refers towhether a defending state’s use of force is commensurate with the use of force in the original armed attack. As a result, jus in bello permits the United States to legally execute a wider range of forcible actions in response to Iranian aggression.

Second, the concept of imminence under jus ad bellum is broader and less restrictive than its ROE counterpart, and is irrelevant if an armed attack is ongoing. There is a lack of consensus regarding how long a victim state may delay prior to using force if an armed attack has occurred. However, concerns over how long a state took to use force in self-defense will likely arise long after the instigating initial attack and are highly fact specific. While Article 51 excludes mention of anticipatory self-defense, most observers agree that states may also use force in anticipation of an imminent armed attack, though the outer limits of this authority are quite contested. In short, significant delay in either direction—either prior to or following an armed attack—will call into question a victim state’s use of force as lawful self-defense. It is not difficult, however, to meet the imminence standard within a pattern of episodic hostilities, wherein an instance of use of force by the victim state could be justified as either a response to an attack or to prevent against a likely follow-on attack.

Last, there is the phrase “armed attack.” Critically, this phrase is exclusive to the jus ad bellum paradigm; therefore, its use in strategic messaging clearly signals a shift away from the U.S. ROE and toward self-defense under the U.N. Charter. The definition of “armed attack” is the subject of continuous debate and litigation. It means something broader than the concept of “aggression” and is narrower and slightly different from Article 2’s prohibition on the illegal “use of force.” But no matter how an armed attack is defined, there are no formalist requirements that the attack use a specific type of weapon or kill a certain number of people, for instance. This low threshold means that the United States may implicate the jus ad bellum framework by simply and explicitly warning Iran that it views certain actions as illegal “uses of force” or “armed attacks” that would, in the U.S. view, trigger its right to self-defense under Article 51 of the U.N. Charter.

If the United States wants to more effectively influence Iranian behavior and curb its aggressive actions, it must inject legal credibility back into its threats. That first requires clearly implicating the jus ad bellum paradigm. This can be accomplished by using the paradigm-specific phrases “use of force” and “armed attack” in U.S. messaging to describe Iran’s actions, or by offering a draft Security Council resolution that characterizes Iran’s actions as a “threat to the peace or breach of the peace.” Unlike confusing U.S.-specific ROE terminology, the international community understands jus ad bellum definitions and standards. The world may in turn assess the legal basis of a U.S. threat to use force by applying jus ad bellum concepts to the facts as perceived by the United States. It does not matter if that assessment results in widespread agreement regarding the propriety of the U.S. interpretation and application of international law. What matters is that Iran is convinced that the United States believes that an armed attack has occurred (or is imminent) and short-of-force avenues are exhausted, thus crossing the threshold and giving the U.S. a valid jus ad bellum basis to use force. As long as the U.S. characterization of Iranian actions and applicable interpretation of international law are reasonable enough, the messaging will have the desired deterrent effect.

Increased Legitimacy

When the United States clearly invokes the jus ad bellum paradigm in its strategic messaging, it implicates terms and standards that increase the credibility of the legal basis for its use of force under international law. In turn, the credible legal basis that results from reasonable interpretation of the facts and application of universally understood legal standards will increase U.S. legitimacy. U.S. military doctrine acknowledges that “legitimacy,” which is rooted in “the actual and perceived legality, morality, and rightness of the actions from the various perspectives of interested audiences,” is a decisive factor in military operations. If one side to a conflict establishes clear legal justification for its actions first, labeling those actions as legitimate becomes much easier for the international community. Conversely, it is rare that illegal actions are perceived as legitimate.

While all sides to a conflict generally seek to assert the legality of their own actions and criminalize those of the enemy, aggressive state actors are far more successful at misusing the law to their advantage. Iran is better at operating in the gray zone, which relies on the use of proxies to conceal state attribution and obfuscate violations of rules and norms, thus making it harder for the victim state to respond. Early formulation of a jus ad bellum narrative justifying force shapes legal arguments in favor of the United States and proactively counters potential Iranian lawfare. This can mitigate after-the-fact unfavorable legal analysis, as evident in the weak legal rationale of the decision in the Oil Platforms Case. Admittedly, the U.S. interpretation of international principles, including those under the jus ad bellum, diverges from those of our international partners. But so long as the U.S. interpretation is reasonable, expressed in good faith and relates to an unsettled concept, a strategic message will carry significant deterrent force.

Sometimes, the United States will obtain information regarding an adversary or foreign operation that justifies the use of force in response and where attribution is unavoidable, but disclosure of that information will compromise a source or method of intelligence gathering. Instead of offering no justification for U.S. action and risking a loss of legitimacy, leaders and policymakers can premitigate the situation by deliberately crafting a narrative that is factually and legal credible. If this narrative is coupled with clear and consistent messages asserting that credible—albeit unreleasable—intelligence existed to justify U.S. action, it will help garner the benefit of the doubt and limit internal confusion. This is far more likely to create a perception of legitimacy than slowly bleeding out contradictory and conflicting narratives that decrease the perceived legitimacy of the use of force, when in fact it may have been perfectly lawful and justified to begin with.

After-the-fact legal posturing to portray the other side as the aggressor is exactly what Iran did when it colorfullydescribed the strike against Maj. Gen. Qassem Soleimani as “assassination,” “terrorism” and “aggression”—then asserted that their own retaliatory strike days later was lawful under Article 51. The fact that modern jus ad bellum tends to benefit aggressive states means the United States must be legally proactive in crafting a unified deterrence strategy, which requires setting the jus ad bellum narrative early and maintaining it consistently. When the United States clearly points to credible bases under jus ad bellum to justify its use of force in self-defense, it will receive greater legitimacy on the world stage, which will further pressure Iran to discontinue its malign actions. This may require the United States to increase transparency by revealing facts that give rise to its legal bases; often, the U.S. relies on classified information to support its use of force. Where revealing limited facts can support its justification to use force under international law—without revealing the means or methods of acquiring such information—the United States should seriously consider doing so.

Escalation Control

An important aspect of effective deterrence is the ability for one side to a conflict to control how and when it escalates. The state that controls escalation can threaten to use force against its adversary in order to secure better terms at the conclusion of the conflict. Controlling escalation also allows the state to retreat from the conflict before it has suffered too heavily. However, escalation control is dangerous because a small miscalculation could lead to armed conflict. If a state determines it has lost or will lose the ability to control escalation, it is less likely to continue its actions lest it trigger an undesirable response. So long as the ROE governs U.S. responses to Iranian aggression, there is little prospect of serious consequences for Iran. Under the ROE, Iran controls escalation by determining when and how to disengage from each separate provocation, which are viewed in isolation. Strategic messaging using jus ad bellum terms and standards signals a shift away from a peacetime paradigm that favors Iran toward a potential armed conflict or jus in bello paradigm that favors the United States, and accordingly may have a powerful deterrent effect. If Iran is aware that its next move may initiate a shift into armed conflict where its forces may be attacked based on their status alone, and not the conduct of each vessel, Iran will recognize it can no longer control the escalation of hostilities and will be more likely to restrict its provocative activities.

The improved clarity produced by unequivocal application of the jus ad bellum paradigm has a somewhat paradoxical secondary effect related to the obscurity of its standards. Once it is clear that the United States is operating under jus ad bellum rules, it benefits from the strategic ambiguity that the jus ad bellum’s broad definitions provide. For example, as previously discussed, jus ad bellum proportionality makes available a wide range of U.S. responses. This means an Iranian miscalculation may invite a response of unpredictable magnitude. Additionally, the United States benefits from the lack of consensus over what constitutes an armed attack and the nebulous definition of imminence as it relates to anticipatory self-defense. If the U.S. clearly invoked the jus ad bellum paradigm, Iran would be unable to predict exactlywhen the United States would deem the threshold into armed conflict crossed but would be aware that the threshold was close. As state practice has shown, ambiguity has worked best when uncertainty surrounds the severity of a response—not the possibility of a response. Such qualified uncertainty would be paralyzing for Iran, which would lose not only its ability to control the escalation of the situation but also its ability to predict when and how the United States would respond altogether. The United States would thus be restored to a position of power from which it could effectively deter Iranian actions.

International Armed Conflict: A Low Legal Threshold

In order for the United States to convince Iran that its actions may trigger an armed conflict—and as a result increase the chances of deterring Iranian action—such a threat must be a possible consequence under international law. Therefore, it is necessary to briefly discuss the law of conflict classification to illustrate that such a narrative is legally and factually reasonable, and thus a credible foundation for a deterrence posture. Jean Pictet’s commentary to the Geneva Conventions reveals how incredibly low a threshold it is to trigger an international armed conflict under Common Article 2:

Any difference arising between two States and leading to the intervention of members of the armed forces is an armed conflict within the meaning of Article 2, even if one of the Parties denies the existence of a state of war. It makes no difference how long the conflict lasts, or how much slaughter takes place.

Under this standard, there is a credible legal argument that the United States and Iran are already in an ongoing state of international armed conflict. The United States hinted as much by justifying the strike against Soleimani as a response to an “escalating series of attacks.” The International Court of Justice appears to have implicitly adopted the practical and legally sound position that a series of hostile actions can cumulatively constitute an armed attack (Oil Platforms, para. 64; and Armed Activities on the Territory of the Congo, para. 147).

Given the escalating series of attacks by Iran and its proxies, followed by the strike against Soleimani on Jan. 3 and Iran’s retaliatory missile attack, it is not specious to conclude from these formulations that Iran’s actions constitute illegal armed attacks and that Iran’s actions have triggered a de jure armed conflict, albeit episodic, between Iran and the United States. Some experts can and do disagree, while others do not. Disparate opinions simply highlight the criticality of precise messaging in U.S. official statements, which must use rules from the correct legal paradigm to show that an undesirable shift from peacetime to armed conflict is actually possible and that the U.S. is committed to crossing the threshold under certain conditions.

A U.S. warning to Iran that its belligerent actions would—in the U.S.’s estimation—initiate an armed conflict is thus a powerful deterrent communication. Such a communication loses its deterrent effect if it implies an armed conflict already exists, in which case the threat to enter one is moot, as an escalation in hostilities is not likely to result. A communication also loses its deterrent effect if, by invoking human rights law terminology, it suggests that the parties remain a safe distance from the threshold of armed conflict. Jus ad bellum is therefore the superior lexicon for strategic communications that strive to fulfill a deterrence strategy. Once more, what matters is that Iran is convinced the United States believes its right to self-defense under jus ad bellum has been triggered, creating an armed conflict wherein the law of armed conflict governs and enemy combatants and military objectives are targetable based on their status. Given the minimal requirements under Common Article 2, it is easy to credibly claim the threshold has been (or will be) crossed, unleashing the favorable jus in bello targeting paradigm for the United States against Iran.

Jus ad Bellum Is the Superior Deterrent Lexicon

Precision in strategic communications is vital when armed parties operate aggressively in close quarters. The United States and Iran are dueling dangerously at the fringes of war, spewing threats and aggressively posturing in a precarious pas de deux. The manner in which the United States frames its threatened response to Iranian actions implicitly communicates the U.S. position as to the existence of, or potential for, armed conflict. If the United States continues to telegraph that the controlling legal framework consists of the limited and reactive peacetime rules under the ROE, Iran will continue undeterred in its actions, emboldened in the absence of an undesirable future consequence and confident in its ability to control the escalation of the conflict. Conversely, if the United States credibly signals that Iranian actions will trigger an armed conflict—even a limited one reminiscent of the Tanker Wars—Iran may restrain its conduct.

Deterrence aims to prevent adversary action by credibly threatening unacceptable counteraction and signaling the willingness to act on those threats. The adversary is deterred from its conduct upon calculation that the costs will outweigh the benefits of continued action. U.S. willingness to use force is dubious when its threats lack a sound justification under international law or feature inconsistent messaging. The United States has long taken the position that the inherent right of self-defense applies against any illegal use of force. By first characterizing Iranian action as illegal uses of force or armed attacks through jus ad bellum terminology, the United States clearly communicates its belief that its right to self-defense under Article 51 will be triggered. Eliminating ambiguity as to which paradigm applies permits the United States to rest its arguments squarely in a reasonable and defensible interpretation of international law, building credibility for a future use of force. Threats derive their credibility from reasonable legal bases, which in turn also enhance the perceived legitimacy of the action once taken. In other words, willingness to act corresponds with legitimacy as bestowed by the international community. With a credible basis for the use of force and prospects that the U.S. position will be seen as legitimate both domestically and internationally, Iran is more likely to believe that the United States is also willing to carry out its threat.

Iran may cease its aggressive actions if it believed they would invite a U.S. response, justified in self-defense under Article 51, that would hurl the parties into a legal state of armed conflict. Without an ability to accurately predict how and where the United States may respond under jus in bello status-based targeting, the stakes for Iran instantly become much higher. Put another way, under the ROE a U.S. response to an aggressive Iranian speedboat must end upon the speedboat’s disengagement, which reduces risk of escalation and gives Iranian forces—in determining when and how to disengage—all of the control. Even if U.S. forces engaged one Iranian speedboat with force, under ROE self-defense, follow-on attacks against other Iranian navy vessels uninvolved in the instant action would be unauthorized absent separate hostile actions or demonstrations of hostile intent. Conversely, jus ad bellum self-defense language signals U.S. contemplation of a shift into a legal state of armed conflict—even if limited—which opens the aperture for lawful attacks against other Iranian warships and lawful targets under jus in bello.

In its public and rightful commitment to use force only when doing so is lawful, the United States must construct threats related to armed conflict under the proper jus ad bellum framework. Failure to do so degrades U.S. credibility, invites criticism from the international community, and promotes the continued ambiguity and legal waffling that have historically been unsuccessful in deterring Iran. Statements by U.S. officials following the April 15 speedboat incident were conflicting and ambiguous, and did not state the circumstances under which the United States would resort to force in response to Iranian naval harassment. These statements also walked back prior U.S. signaling that Iran was close to triggering a de jure international armed conflict. Uncommitted explanations and general wavering simply embolden Iran. Iran’s recent successful and 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 first demonstration was unsuccessful, the timing of its execution suggests that Iran does not perceive U.S. threats to be credible.

Jus ad bellum offers states a broad framework to counter aggressive acts with threats and uses of forces lawfully, and its terminology provides government officials and policymakers with a rich legal vocabulary with which to paint a clear, credible and costly deterrent picture, forcing potential adversaries to weigh their actions carefully before rolling the iron dice. Shaping the narrative in legally favorable terms under international law preserves the United States’s reputation and relationships with its allies, making a future collective response to the adversary’s actions appear more likely, as well. Even where the U.S. interpretation of international terms and concepts is vulnerable to disagreement, so long as its interpretations are articulated in a consistent manner and are perceived as made in good faith, the desired deterrent effects will still result. This is because it is ultimately the U.S. view that drives the U.S. response: So long as the enemy—Iran, here—believes that the U.S. is convinced of the merit in its own basis to use force, it may likewise be convinced that the U.S. will actually act. Because the United States has the military and intelligence capacity to inflict significant damage to Iran’s Navy and other potential military assets, Iran must carefully reassess whether it is willing to risk a multidimensional attack against its military forces and their leaders in armed conflict, even in a limited way. All of these factors, when harmonized across all the pillars of U.S. strength and messaging, combined with undeniable U.S. capacity to take action, produce a formidable deterrent message tailored to the specific target. With Iran as an example, this threat runs directly counter to Iran’s foreign policy objectives of avoiding escalation and an open conventional conflict with the United States—an objective both countries hold.

By consistently relying on the jus ad bellum paradigm and lexicon, the United States can communicate that it perceives itself as the victim of illegal aggression and armed attacks by Iran—and that Tehran’s actions are perilously close to initiating an armed conflict with the United States and triggering the jus in bello. With the consequences clarified and the basis for the use of force rooted in a credible interpretation of international law, the U.S. will improve the clarity and increase the legitimacy of its threats. By demonstrating a reluctant but legal willingness to decisively use force in an open and conventional conflict with Iran, the United States can thereby restore its escalation control. Conversely, the U.S. should steer away from the confusing ROE language or mixed paradigms that limit its use-of-force options and allows Iran to retain control of escalating hostilities. A threat to use force in self-defense under the ROE, or only in a tit-for-tat manner, implies that the U.S. perceives Iranian armed attacks as finite and isolated events, far from the threshold of an armed conflict. This is exactly where Iran wants to keep hostilities—to its benefit and the U.S.’s disadvantage.

Matthew J. Aiesi is a Major in the United States Army. He in as an Associate Professor in the National Security Law Department at The Judge Advocate General's Legal Center and School in Charlottesville, Va. The views expressed here are his personal views and do not necessarily reflect those of the Department of Defense, the United States Army, The Judge Advocate General's Legal Center and School, or any other department or agency of the United States Government.
Amanda L. Minikus is an active duty Marine judge advocate and graduate of the U.S. Naval Academy (2006). She holds a J.D. from Cornell University (2015), an LL.M. in International Law and Comparative Law from Cornell University (2015), and an LL.M. in Military Law from TJAGLCS (2020). The views expressed here are her personal views and do not necessarily reflect those of the United States Marine Corps, Department of Defense or any other department or agency of the U.S. Government.

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