On April 15, eleven Iranian Islamic Revolutionary Guard Corps (IRGC) Navy gunboats repeatedly traversed the bows and sterns of six U.S. Navy vessels that were conducting joint integration operations with U.S. Army Apache attack helicopters in international waters of the Persian Gulf. The U.S. Navy claimed the gunboats’ repeated crossings at extremely close range, one as close as 10 yards, were “dangerous and harassing approaches.”
In response, President Trump instructed the Navy to “shoot down and destroy” IRGC Navy gunboats that “harass” U.S. ships. This was his most direct threat of military action against Iran since authorizing the targeted killing of Major General Qassem Soleimani, the former military commander of the IRGC’s Quds Force. In a briefing at the White House, the president claimed, “we’re covered 100 percent” with respect to the U.S. military’s current rules of engagement.
In this article, we assess whether U.S. warships have a right under international law to use force in self-defense against IRGC gunboats engaged in harassment operations. In particular, we examine the U.S. military’s Standing Rules of Engagement, which provide the operational architecture for defensive action by U.S forces, including warships at sea.
We do not address international humanitarian law issues, which regulate state behavior once they are locked in an armed conflict, beyond noting that hostilities between the United States and Iran would, indeed, initiate an international armed conflict and the gunboats would be lawful targets under that body of law, for they are military objectives by nature. This would be so even if the U.S. warships did not enjoy a right to employ force in self-defense under the body of international law that governs the resort to force by states (jus ad bellum).
Use of Force in Self-Defense
To be lawful, any use of force by U.S. forces against Iranian military assets must comply with the law on the use of force, the jus ad bellum. Absent a UN Security Council resolution under Chapter VII of authorizing the action, engaging IRGC Navy gunboats would have to be based on the law of self-defense found in Article 51 of the U.N. Charter and its customary international law counterpart. Article 51 provides, “Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.” 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 and customary international law.
In interpreting Article 51 and customary international law, the United States has taken two positions of particular relevance to the gunboat scenario. First, it has rejected the prevailing view among states and scholars that, as articulated by the International Court of Justice in its Military and Paramilitary Activities in and against Nicaragua judgment, the category of “armed attack” is limited to “the most grave forms of the use of force.” Rather, the United States “has long taken the position that the inherent right of self-defense potentially applies against any illegal use of force.” In other words, so long as the Iranian actions qualify as a “use of force,” the United States is of the opinion that it may lawfully respond with its own use of force in self-defense.
This is a view with which one of us disagrees. That said, in its Oil Platforms decision, the International Court of Justice 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-defence,’” and this is particularly relevant to the case at hand. In our opinion, an attack by the military forces of one country against a warship of another certainly amounts to an armed attack under the law of self-defense irrespective of the position taken on whether there is a “gap” between the use of force and armed attack thresholds. In other words, such an attack would trigger the right to use force in self-defense under either the U.S. view (that any uses of force can trigger the right of self-defense) or the prevailing view (that only the most grave forms of the use of force do).
Second, although on its face Article 51 applies only to an ongoing armed attack, the United States has long contended, correctly so, that it also applies anticipatorily — that is, states enjoy the “right to take measures in response to imminent attacks.” 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. The clearest and most up-to-date articulation of the U.S. interpretation of “imminent” is contained in the White House’s 2016 Legal and Policy Frameworks report.
To determine whether an armed attack is imminent, the United States considers a variety of factors, including “the nature and immediacy of the threat; the probability of an attack; whether the anticipated attack is part of a concerted pattern of continuing armed activity; the likely scale of the attack and the injury, loss, or damage likely to result therefrom in the absence of mitigating action; and the likelihood that there will be other opportunities to undertake effective action in self-defense that may be expected to cause less serious collateral injury, loss, or damage.” These factors are most likely to be analyzed at the national level in order to authorize national self-defense, as opposed to unit or individual self-defense. From the perspective of the commander making the decision at the unit level when time is of the essence, a more manageable test is whether the defensive action is taking place “during the last possible window of opportunity in the face of an attack that was almost certainly going to occur.”
However, finding that an armed attack is ongoing or imminent is not sufficient to deem the use of force in self-defense lawful. As noted by the International Court of Justice in cases such as the Nuclear Weapons Advisory Opinion, Armed Activities, and Oil Platforms, defensive uses of force must be both necessary and proportionate. The United States shares this view.
One last point deserves mention for the specific topic that we’re analyzing here. International law prohibits the “the threat or use of force” unless in self-defense. In other words, if the use of force were unlawful in a specific situation, threatening to use force in that situation would also be unlawful.
Operationalizing the Law of Self-Defense: The Standing Rules of Engagement
To operationalize the law of self-defense, the United States has promulgated the Chairman of the Joint Chiefs of Staff’s Standing Rules of Engagement (SROE), the most recent version of which was issued in 2005. As a general matter, rules of engagement are “[d]irectives issued by competent military authority that delineate the circumstances and limitations under which United States forces will initiate and/or continue combat engagement with other forces encountered.” The SROE are “standing” rules in the sense that they remain in force during all military operations and contingencies occurring outside U.S. territory, as well as during air and maritime homeland defense missions in U.S. territory, national airspace, or territorial seas, unless otherwise directed by the secretary of defense.
The SROE translate ongoing and imminent armed attacks into the operationally understandable terms “hostile act” and “hostile intent.” The former is “[a]n attack or other use of force against the United States, US forces or other designated persons or property. It also includes force used directly to preclude or impede the mission and/or duties of US forces, including the recovery of US personnel or vital USG property.” The reference to “attack or other use of force” satisfies the armed attack requirement, at least for the United States because it equates all uses of force with an armed attack.
Hostile intent is the SROE’s operational version of anticipatory self-defense. It defines the situation as involving “[t]he threat of imminent use of force against the United States, US forces or other designated persons or property. It also includes the threat of force to preclude or impede the mission and/or duties of US forces, including the recovery of US personnel or vital USG property.” As with “hostile act,” the definition comports with the condition precedent of an armed attack, at least by the U.S. understanding of that threshold.
The determination of imminence is always contextual; it is “based on an assessment of all facts and circumstances known to US forces at the time.” Importantly, the 2005 SROE added the point that imminent “does not necessarily mean immediate or instantaneous,” likely in response to the so-called Bush Doctrine set forth in the 2002 National Security Strategy. This addition in no way is meant to suggest that so-called “preventive” self-defense is authorized. Rather, it simply acknowledges that the commander need not wait until the attack is on the cusp of execution before defending the unit; there is no “don’t fire until you see the whites of their eyes” requirement in anticipatory self-defense.
The SROE also incorporates the principles of necessity and proportionality into its analysis. In the operational context, military personnel are trained that they do not have to “take the first hit” before defending their unit. Lest this statement be read too broadly, the SROE caution that self-defense is permitted only “while the force continues to commit hostile acts or exhibit hostile intent.” Further, as reflected in the Department of Defense Law of War Manual “no reasonable alternative means of redress [can be] available” to the commander facing the demonstration of hostile intent. This reflects the classic understanding of the necessity criterion.
With respect to proportionality, the SROE accurately describe a proportionate response as a use of force that is “sufficient to respond decisively to hostile acts or demonstrations of hostile intent.” It “may exceed the means and intensity of the hostile act or hostile intent, but the nature, duration and scope of force used should not exceed what is required.” The DOD Law of War Manual takes the same approach when it notes that a response to an armed attack is proportionate “to the extent that it is required to repel the armed attack and to restore the security of the party attacked.” Simplified, the principles of necessity and proportionality disallow the use of force when non-forceful alternatives likely to suffice exist, when the armed attack is not yet imminent, and when the attack is clearly over and not one in a series of attacks constituting a campaign.
The SROE may be “supplemented” with tailored “mission-specific” rules of engagement to facilitate the accomplishment of particular operations or missions, including during actual hostilities. Current mission-specific rules of engagement are classified because they would reveal U.S. tactics, techniques, and procedures (so called “TTP)” to those against whom force might be used. (Historic ROE have been released.) An example would be declaring a particular group as “hostile,” thereby allowing U.S. forces to engage individuals on the basis of their status as members of the designated group. However, since such “status-based targeting” is permissible only during an armed conflict, a rule to this effect for the IRGC Navy gunboats or other Iranian forces would be unlawful unless and until the two countries found themselves in an armed conflict.
Categories of Self-Defense in the SROE
The SROE recognize three categories of self-defense — national, unit, and individual. The primary difference lies in the level of authority and responsibility at which the right of self-defense is exercised. National self-defense is “[d]efense of the United States, US forces, and, in certain circumstances, US persons and their property … from a hostile act or demonstration of hostile intent.” Unit commanders “may exercise National Self-Defense, as authorized” (emphasis added).
By contrast, unit commanders always have the “inherent right and obligation to exercise unit self-defense in response to a hostile act or demonstrated hostile intent.” This right applies to their unit, warship, or aircraft, or those forces with which they are presently operating.
Despite assertions that unit self-defense has a legal basis distinct from that of national self-defense, we agree with Yoram Dinstein’s observation in his classic work, “War, Aggression and Self-Defence,” that there is “a quantitative difference but no qualitative difference between a single unit responding … and the entire military structure doing so.” The legal basis for both responses is the international law right of self-defense in the face of an armed attack. Unless otherwise directed by a unit commander, the SROE also authorize “military members [to] exercise individual self-defense in response to a hostile act or demonstrated hostile intent” against them. When assigned to and acting as part of a unit, such individual self-defense is considered as a subset of unit self-defense and therefore has the same basis in the international law of self-defense.
Unit Self-Defense Requirements
Given that the international law of self-defense is the basis for these three categories, unit self-defense taken pursuant to the SROE is authorized only in the face of ongoing or imminent armed attack, which is defined in the SROE as a hostile act or hostile intent, and it must meet the requirements of necessity and proportionality.
At the unit level, the SROE’s de-escalation principle is designed to satisfy the necessity criterion. It provides, “[w]hen time and circumstances permit, the forces committing hostile acts or demonstrating hostile intent should be warned and given the opportunity to withdraw or cease threatening actions.” Of course, whether forces are demonstrating hostile intent and whether de-escalation should be attempted can be an extraordinarily difficult decision for a commander.
Consider a costly incident involving a commander’s discretion to employ force in unit self-defense. In 1987, during the Iran-Iraq War, 37 U.S. Navy personnel were killed when an Iraqi Air Force jet launched two anti-ship missiles against the USS Stark. The ship had identified the Iraqi jet approximately 70 miles out and transmitted warnings as it approached. An investigation by the House of Representatives Armed Services Committee determined that the requirement for de-escalatory measures prior to the exercise of self-defense likely factored into the commander’s hesitancy to defend his or her ship.
By contrast, in 1988, near the end of that war, the USS Vincennes, a guided-missile cruiser, shot down Iran Air Flight 655, a civilian passenger aircraft that had taken off from Bandar Abbas International Airport but was mistakenly identified as a combat aircraft operating in an attack profile. Two hundred and ninety people on board perished. And this year, Iran mistakenly downed a Ukrainian International Airlines flight killing 176 people during a period of heightened tension between the United States and Iran following the targeting of Soleimani and threats of military action by both sides.
The point here is simply to underscore the complex nature of these decisions, especially in exigent situations that allow little moment for deliberation.
The SROE are equally clear that self-defense must be proportionate. Accordingly, even in cases where a commander has concluded his or her forces are under attack, or about to be attacked, and the use of force is necessary, the SROE caution that the “the nature, duration, and scope of [defensive] force should not exceed what is required.” Using more force than required to effectively deter or turn back an attack would amount to an unlawful use of force by the United States against Iran. Of course, it is difficult to calculate the precise degree of force necessary to do so and therefore there is a fair margin of appreciation with respect to the commander’s decision in that regard.
Responding to Iranian Harassment
The U.S. warships did not fire on the IRGC gunboats even though one came within 10 yards of a warship. That was proper according to the SROE and international law. The gunboats were not attacking the warships or the Apache helicopters; they had not engaged in a “hostile act” as that term is understood in the SROE.
The same follows from the U.S. commander’s calculated decision whether to characterize the Iranian operations as a demonstration of “hostile intent,” thereby permitting a response in anticipatory self-defense pursuant to the SROE. The gunboats were armed, and friction between the United States and Iran was acute. Nevertheless, the commander concluded that considering all attendant circumstances, probably including prior harassment incidents, the gunboats were harassing the U.S. forces but not about to attack them. In retrospect, that was a sound call; and, indeed, no attack resulted.
Turning to the president’s directive to “shoot down and destroy” IRGC Navy gunboats, whether the U.S. Navy will be justified in destroying gunboats on the basis of unit self-defense will depend on the specific circumstances. The commander must conclude that a hostile act has occurred or that the gunboats have demonstrated hostile intent, that employing force against the gunboats is the only way to defeat the attack, or disable once that is imminent, in the circumstances, and that the degree of force used is limited to that needed to achieve do so.
Short of a demonstration of hostile intent or a hostile act by Iranian gunboats against U.S. assets that triggers the right of unit self-defense, the sole basis for engaging them would be national self-defense. This would require an unrelated armed attack against the United States or U.S. forces and a determination that, although any targeted Iranian gunboats were not participating at the moment, they would be used in the ongoing Iranian armed attack on the United States. That is not the situation in the Persian Gulf today.
Finally, what about the U.S. Navy’s claim that the Iranian gunboats’ activities involved “dangerous and harassing approaches;” could that endangerment alone constitute a use of force? All hostile intent and hostile act determinations are contextual in applying the SROE, but in most situations such activities would not rise to the level of an imminent armed attack, and would not qualify as a demonstration of hostile intent without an indication that the gunboats were about to use their weapons against the U.S. warships.
Saber rattling has its place. But it is essential that any such rattling not threaten unlawful use of force. As discussed, such threats are themselves an internationally wrongful act in violation of Article 2(4) of the U.N. Charter, which requires member states to “refrain in their international relations from the threat or use of force.”
There are situations in which using force against IRGC Navy gunboats and other Iranian military assets would be lawful under the law on the use of force. These are clearly set forth in the SROE, which have served U.S. forces well in situations short of armed conflict for decades. To the extent the president’s comments were meant to be a warning that U.S. military units will avail themselves of their rights under the law of self-defense as captured in the SROE, they are lawful. We offer no opinion on the issue of whether the comments were well-advised.
However, harassment that does not clearly risk imminent harm to life or certain property does not open the door to self-defense under international law. If the president was threatening that U.S. forces may engage Iranian military assets at a threshold below an imminent or ongoing armed attack, the threat itself violated the U.N. Charter and its customary international law analogue. Any action carrying out that threat would likewise be unlawful. We can only hope it was not his intent to threaten or authorize action inconsistent with the SROE.
Image: Navy Petty Officer 2nd Class Hao Lienh stands watch on a Mark VI patrol boat before a weapons sustainment exercise in the Persian Gulf, April 16, 2020. Photo by Navy Petty Officer 1st Class Kory Alsberry/U.S. Department of Defense.