On June 20, U.S. military officials confirmed media reports that Iranian military forces successfully shot down a U.S. drone in the vicinity of the Persian Gulf. Though several relevant details regarding the incident remain unclear or in dispute—including whether the drone had ventured into Iranian airspace before it was shot down—this is the most significant encounter between the two sides in the current period of high tensions and may prove to be a major step toward a broader armed conflict. And for the United States, it raises the more immediate legal question of whether and how President Trump may choose to respond militarily.

Domestic Law

From the perspective of U.S. domestic law, executive branch lawyers have long maintained that the president has the authority under Article II of the Constitution to “take military action ... for the purpose of protecting important national interests, even without specific prior authorization from Congress … at least insofar as Congress has not specifically restricted it[.]” As Congress as a body has thus far remained silent on possible military action against Iran—potential arguments regarding the inapplicability of the 2001 Authorization for Use of Military Force notwithstanding—Article II is the most likely framework through which the executive branch will gauge the president’s authority to respond militarily to this latest incident. In laying out the legal justification for the Trump administration’s May 2018 airstrikes against Syria, the Justice Department’s Office of Legal Counsel (OLC) used a two-part test to determine where and how this authority may be used, stating:

First, we consider whether the President could reasonably determine that the action serves important national interests. Second, we consider whether the “anticipated nature, scope and duration” of the conflict might rise to the level of a war under the Constitution [that would require congressional authorization].

There is little doubt that the drone shootdown satisfies the first prong of this test. Prior OLC opinions identify defending against attacks on U.S. government property as a sufficient “national interest” to warrant the use of military force. Moreover, Iran’s broader conduct arguably threatens other national interests recognized by the OLC as well, including responding to threats to U.S. personnel, promoting regional stability and deterring the use of weapons of mass destruction. And the OLC’s 2018 Syria opinion makes clear that this is a highly deferential standard as, in its view, “[t]hese interests understandably grant the President a great deal of discretion.”

The second prong may bar the Trump administration from committing the United States to a large-scale ground invasion or other extended military campaign absent congressional consent. But it poses few barriers to retaliatory airstrikes or other more limited types of responses. The Syria opinion does emphasize the risk of escalation as an important factor in this latter analysis—and such a risk is no doubt a concern with regard to Iran, given the regional positioning and hostile rhetoric that both sides have pursued. For this reason, executive branch lawyers may well advise the Trump administration to take steps to mitigate some of these risks, including by setting express limits on the military response pursued, tailoring it to specific strategic objectives and taking steps to minimize unnecessary casualties. That said, the OLC is likely to defer to policymakers on relevant fact and policy judgments, including the likelihood of escalation and what sort of military action is necessary to accomplish certain objectives.

While Congress has not weighed in on the conflict with Iran, it has set certain general restrictions on how the president may use Article II authorities. Specifically, the 1973 War Powers Resolution directs the president to “in every possible instance … consult with Congress” before and after any use of military force. It also requires that the president notify Congress within 48 hours whenever the “United States Armed Forces are introduced ... into hostilities” and withdraw them within 60 days (extendable to 90 days) unless they receive congressional authorization. The executive branch has generally viewed the consultation requirement as advisory due to constitutional objections, but has—with one early exception—accepted the notification and 60- to 90-day requirements as valid in most circumstances. This means that Congress may not have the opportunity to provide advance input regarding a possible retaliation against Iran but should receive official notice if and when such retaliation occurs. As for the 60- to 90-day time limit, the executive branch has interpreted “hostilities” narrowly to mean situations in which “units of the U.S. armed forces are actively engaged in exchanges of fire with opposing units of hostile forces[,]” which has allowed certain lower-intensity military campaigns to continue past this time frame even without congressional authorization. Depending on the type of military response the Trump administration pursues, it may be able to deploy a similar argument in the event operations last beyond 90 days.

Of course, this assumes the Trump administration will apply the same framework to a conflict with Iran that it applied to the 2018 Syria strikes—and there is no guarantee that it will. As Jack Goldsmith has written for Lawfare, the executive branch has at times advanced even broader views of the president’s legal authority, particularly in situations of national self-defense. Embodied in a pair of extant (if rarely cited) post-9/11 OLC opinions, this perspective would give the president even more discretion in deciding when and how to use military force and is less accepting of statutory limitations imposed by Congress. Moreover, even if the OLC felt bound by the 2018 Syria opinion and its predecessors, the president and the attorney general have the authority to override or reject OLC opinions. Attorney General William Barr, at least, has hinted that he has an exceptionally broad view of the president’s authority to use military force. Openly embracing such a view may come with substantial political costs, especially given the controversy that still surrounds OLC opinions from the immediate post-9/11 era. But the Trump administration may see the trade-off as worthwhile to ensure its operational flexibility or calculate that the trade-off is unlikely to be of concern to core political constituencies.

None of this means that the executive branch’s reading of the law is necessarily correct, regardless of which view the Trump administration embraces. Many critics object that the executive branch routinely discounts Congress’s own constitutional war powers and gives the president more authority to use military force than the law actually allows. But due to the president’s role as commander in chief, the executive branch’s view is the one that will guide any U.S. decision regarding a possible military response. And unless the federal courts overcome their usual reticence to involve themselves in such matters—or Congress enacts substantial limitations into law, a possibility currently being debated—it is likely to remain the operational one for the course of whatever conflict follows.

International Law

International law, meanwhile, raises a separate set of legal considerations. The extent to which these international legal considerations bear on the president’s constitutional authority to use military force is contested: While some observers maintain that U.S. international legal obligations constrain the president through his constitutional obligation to “take Care that the Laws be faithfully executed[,]” the OLC has argued—most recently in a 1989 opinion written by Barr, which reversed an earlier opinion—that it does not. Regardless, if the Trump administration wishes to make clear to the international community that it is responding to Iran in a manner consistent with U.S. international legal obligations, then it will need to take the international lawfulness of any military response into account. And the lawfulness very much depends on whether the U.S. drone was in Iranian airspace or international airspace.

If the U.S. drone really had been in Iranian airspace, it seems permissible under international law for Iran to shoot it down, at least if Iran had first attempted to warn the drone or its operator. It is a basic principle of sovereignty that a state has the right to control access to its territory, airspace and territorial waters. Article 1 of the Chicago Convention on International Civil Aviation, to which the United States and Iran are both parties, provides that “[t]he contracting States recognize that every State has complete and exclusive jurisdiction over the airspace above its territory.” Article 2 clarifies that “territory” constitutes “the land areas and territorial waters adjacent thereto.” The Chicago Convention also deals specifically with unmanned aircraft. Article 8 of the Chicago Convention states, in relevant part, that “[n]o aircraft capable of being flown without a pilot shall be flown without a pilot over the territory of a contracting State without special authorization by that State and in accordance with the terms of such authorization.”

Iran therefore is entitled to control access to its airspace. The relevant question here is whether Iran can use force against an aircraft that has entered its airspace without consent. Although there is no black letter law on the question, state practice suggests that a state can use force against unmanned drones that have entered its airspace without consent. If the military aircraft were manned, and the involved states were not in an armed conflict, there appears to be an expectation that the territorial state will give warnings before using force against it. For example, U.S. fighter jets fired warning shots at Russian planes flying through coalition airspace in Syria in 2017. This is consistent with Department of Defense Instruction 4540.01 (2015) on the Use of International Airspace by U.S. Military Aircraft and for Missile and Projectile Firings. Section 3(e) states, “U.S. military aircraft and missile and projectile firings operate with due regard for the safety of all air and surface traffic,” and notes that “[r]easonable warning procedures with regard to the military aircraft of all States must be observed.” It also is a sensible rule, as there are occasions on which a military aircraft enters another state’s airspace in error or because the plane is in distress. (We welcome additional information about Defense Department rules or policies about the interception of military aircraft.)

It is less clear whether states can or will warn military drones before shooting them down. In recent years, Japan, Israel and India have pursued policies reflecting a view that it is lawful for a territorial state to shoot down a drone that enters its airspace, but not all of those states seem to have warned first.

Japan has a policy of trying to warn the drones first. In 2013, Japanese Prime Minister Abe approved a plan to shoot down foreign drones flying in Japanese airspace if the drones (or, presumably, their operators) ignore warnings to leave. Japan developed this policy in the wake of Chinese drone flights over the Senkaku/Diaoyu islands, the ownership of which Japan and China contest. According to the Japanese Times, the Japanese government determined that Chinese drones are equipped with high-performance cameras that are capable of detecting warnings.

Israel has shot down drones that entered or approached its airspace. In 2014, during the Gaza war, Israel used a Patriot missile to destroy an unmanned Hamas aircraft over the Israeli city of Ashdod. In February 2017, an Israeli fighter jet shot down a Hamas drone that took off from the Gaza Strip, hitting it before it entered Israeli airspace. In that case, the military stated that the interception prevented an “immediate threat of infiltration.” And in November 2017, Israel shot down a drone that tried to infiltrate its airspace from Syria. It intercepted the drone with a Patriot missile while it was over the Golan Heights. As for India, in 2019 an Indian air force jet shot down what reportedly was a Pakistani drone in Indian airspace. In the Israeli and Indian situations, it is unclear whether the states using force first tried to warn the drones or their operators (if known).

This leaves unresolved whether there is an international legal requirement for a territorial state to give a warning to a military aircraft that has entered its airspace when the territorial state is not engaged in an armed conflict and there is no reason to believe the aircraft poses an imminent threat of an armed attack. It appears to be U.S. policy to warn where possible, and other states may take this approach as well. There is no indication in press reports that Iran attempted to warn the U.S. drone or its operators that it had entered Iranian airspace. Further, nothing in Iran’s rhetoric indicates that it deemed the drone flight to be an armed attack that triggered its right of self-defense and rendered it necessary to use force.

If the U.S. drone was in international airspace and was not engaged in hostile military activities, however, Iran did indeed engage in an internationally wrongful act by shooting it down. Most observers would conclude that, on its own, this action would only warrant U.S. responses short of using military force, such as lawful countermeasures. To warrant a U.S. military response, Iran’s actions would have to trigger the inherent right to self-defense referenced in Article 51 of the U.N. Charter, which is an exception to the prohibition on the use of force between states in Article 2(4) of the same. Those states that abide by the standard set forth in the International Court of Justice’s Nicaragua decision may have doubts as to whether an attack on an unmanned drone is one of “the most grave forms of the use of force” necessary to trigger the right to self-defense. But the United States has openly objected to this standard and argued that the right can be triggered by any use of “deadly force by the military personnel of another state[,]” including in cases that resulted in no fatalities. In the present case, the fact that the Iranian attack could not under any scenario have resulted in fatalities—because the drone was unmanned—no doubt weighs against a U.S. claim of self-defense and presents what is, to our knowledge, a question of first impression. That said, it is still undoubtedly a foreign military attack on a U.S. aircraft engaged in lawful activities in international airspace, which the U.S. executive branch, at least, seems likely to find sufficient to trigger a forcible response.

This does not, however, provide the United States with a blank check. As U.S. military doctrine acknowledges, any response will have to satisfy a variety of international legal requirements, including those of necessity and proportionality under the jus ad bellum, as well as distinction, necessity and proportionality under the jus in bello. In short, this means any response will have to be pursuant to a legitimate military need, employ only a level of force proportional to the military advantage provided, and scrupulously minimize damage to nonmilitary targets, including civilian casualties. Implementing these obligations necessarily entails a substantial amount of judgment by military personnel, which can lead to disagreements. Nonetheless, there is no debate that international law sets important limits on how the United States may pursue a military response.

Next Steps

Just because the Trump administration can make a legal case for taking military action against Iran does not mean it will or should use it. As one of us (Deeks) has described, the United States has acted with restraint toward other attempts to shoot down U.S. drones. Nor has the Trump administration always felt the need to respond with military force when U.S. drones have been shot down in Syria and Yemen. And however provocative its recent actions have been, Iran has steered conspicuously clear of the red line that Secretary of State Mike Pompeo reportedly drew in May: namely, that even a single U.S. casualty would trigger a military response. As Trump himself noted, it would have made a “big, big difference” if U.S. personnel were somehow injured in the attack—but they were not.

A military response is, however, a very real possibility. As interpreted by the executive branch, U.S. domestic law gives the president broad leeway in responding to overseas attacks on U.S. personnel and property. And while much of the international community will likely have doubts as to whether this most recent incident provides a basis for using military force under international law, the United States likely believes that it does. To the extent that the executive branch sees legal constraints on its options, such constraints are most likely to limit the type of response, as mainstream views of both domestic and international law put limits on the military action that the United States can pursue. The most significant constraints on the president, however, are likely to be political and strategic in nature, not legal.