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For the past two months, the Middle East has teetered on the edge of war. Tensions over the U.S. “maximum pressure” campaign have led Iran to target maritime shipping in the Persian Gulf, launch rockets on U.S. diplomatic and military personnel in Iraq through proxies, and shoot down a U.S drone that may or may not have been operating in Iranian airspace. The Trump administration has asserted that it is “considering a full range of options” in response to Iran’s actions, with Secretary of State Mike Pompeo warning Iranian officials (through Iraqi intermediaries) that even a single U.S. fatality would trigger a military response and President Trump briefly authorizing retaliatory airstrikes before calling them off. But U.S. officials have thus far refused to identify the legal basis for a military response. “We always have the authorization to defend American interests," Pompeo asserted in a recent interview, hinting at a possible constitutional argument. Members of Congress, however, have reportedly been briefed that the Trump administration believes Iran can be targeted under the 2001 Authorization for Use of Military Force (AUMF), which is currently used to authorize military operations against al-Qaeda and its affiliates.
Whatever legal theory it embraces, the Trump administration will not be writing on a blank slate. Executive branch lawyers have spent decades articulating theories regarding the president’s legal authority to use military force, some of which the Trump administration has already embraced. Federal courts, meanwhile, have proved reticent to engage legal questions regarding the scope of the president’s war powers, insulating these theories from any meaningful judicial review. As a result, while not necessarily authoritative or correct, the executive branch’s view of the law will almost certainly be the operational one for any U.S. decision to enter into—as well as the subsequent conduct of—hostilities with Iran. And examining these legal theories may in turn provide insight into what military options the Trump administration views as legally available and the conditions under which they may be deployed.
The President’s Constitutional Authority
The executive branch has long maintained that the president can use military force to protect certain U.S. national interests without specific prior authorization from Congress. The Justice Department’s Office of Legal Counsel (OLC) roots this authority in Article II of the Constitution, which makes the president the “Commander in Chief of the Army and Navy” and gives him “the executive Power[,]” as well as the “historical gloss” that decades of congressional acquiescence have put on those authorities. But this authority isn’t unlimited. The OLC has recognized that Congress can impose certain statutory restrictions on the president’s use of military force, as Article I of the Constitution gives Congress the authority to “to declare War[,]” “raise and support Armies[,]” and “provide and maintain a Navy[,]” among other war powers. And, in at least some circumstances, Congress’s own Article I authority sets limits on the size and scale of armed conflict that the president can enter into even before he requires authorization from Congress.
During the Obama administration, the OLC began to reduce its analysis into a two-part test to determine whether a president has the authority to pursue a given military operation. The Obama White House later included this test in a 2016 report it published on legal frameworks for the use of military force. When the Trump administration filed its supplement to this report in March 2018, as required by Congress, it left most of the Obama administration’s legal analysis in place, including this test. And when the OLC later articulated the legal basis for the May 2018 airstrikes against Syria, it applied this test, stating:
In evaluating whether a proposed military action falls within the President’s authority under Article II of the Constitution, we have distilled our precedents into two inquiries. 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.
Thus far, this is the only public statement the Trump administration has issued on when it believes the president may use military force under his own authority. For this reason, this test—and certain other legal constraints reflected in the 2016 report, which the Trump administration has thus far tacitly accepted—is a good place to begin in understanding the Trump administration’s views on when the president can use military force.
As the 2018 Syria opinion concedes, the “national interests” prong of the OLC’s two-part test is not particularly constraining. As construed in the Syria opinion, it requires only that there be some reasonable basis for the president to determine that the military operation in question advances U.S. national interests. Prior OLC opinions have recognized several such interests as warranting the use of force, including protecting U.S. persons and property, protecting foreign persons and property, providing assistance to allies, upholding the credibility of the U.N. system, promoting regional stability, and mitigating humanitarian disasters. Moreover, as the OLC views “evaluat[ing] the worth of the interests at stake” as “a question of policy more than law[,]” this list is representative, not exhaustive. The Syria opinion itself adds deterring the use of chemical weapons to this list by noting U.S. policy statements and analogizing it to other national interests that have served as a basis for using military force in the past. Presumably future presidents could do the same with other national interests.
The “nature, scope and duration” prong of the OLC test, however, may impose more substantial limits. Consistent with prior OLC opinions, the Syria opinion focuses its analysis on whether the proposed operation will involve “prolonged and substantial military engagements [that] expos[e] ... U.S. military personnel to significant risks over a substantial period” and entails inflicting substantial casualties on the enemy. One significant factor in this analysis is whether the military action in question involves the deployment of ground troops, which the Syria opinion recognizes as “an essentially different, and more problematic, type of intervention” than those not involving ground forces due to “the difficulties of disengaging ground forces from situations of conflict ….” Another is “the risk that an initial strike could escalate into a broader conflict[.]” In its Syria opinion, the OLC concluded that the Syria airstrikes mitigated this latter risk by being narrowly tailored to a specific objective, designed to minimize unnecessary casualties, and channeled through existing deconfliction procedures with Russian forces also present in Syria. This leaves the president with substantial discretion in employing targeted airstrikes and other types of limited military action that involve a minimal threat to U.S. personnel. But it also raises doubts about the president’s authority to unilaterally initiate large-scale or enduring hostilities, particularly when they entail the deployment of ground troops—or to take actions that seriously risk involving the United States in such hostilities.
Congress has also put statutory limits on the president’s constitutional authority to use military force, specifically in the form of the 1973 War Powers Resolution. The most relevant provision obligates the president to notify Congress whenever U.S. armed forces are “introduced … into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances” and directs him to withdraw them within 60-90 days unless Congress authorizes such action or cannot meet due to an armed attack. While President Nixon claimed this provision was unconstitutional in his unsuccessful veto of the War Powers Resolution, subsequent OLC opinions have generally accepted it as valid in most circumstances. That said, the executive branch has argued that these provisions are triggered only when “units of the U.S. armed forces are actively engaged in exchanges of fire with opposing units of hostile forces” or “there is a serious risk of hostile fire[,]” meaning they exclude “irregular or infrequent violence[,]” “sporadic military or paramilitary attacks[,]” and actions taken strictly in self-defense. Based on these distinctions, presidential administrations have controversially argued that certain military operations in Lebanon, Libya and Yemen—among other places—could continue past the 60- to 90-day time period without congressional authorization.
What might these factors mean for U.S. military action against Iran? The Trump administration will have little difficulty pointing to national interests that meet the OLC’s standard for the use of force: For decades, U.S. officials have routinely accused Iran of destabilizing the region; Iran and its agents have recently targeted American personnel and property; and the U.S. has long-standing policy interests in both preventing the proliferation of nuclear weapons and ensuring freedom of maritime navigation. But the logic embraced by the Syria opinion puts some outside limits on the type of military action the president may pursue. Trump will have the strongest legal grounds for ordering targeted airstrikes and other limited operations that are narrowly tailored to specific strategic objectives and do not put U.S. personnel in harm’s way. While the president can no doubt make substantial departures from this model, each step will bring him closer to impermissibly infringing on Congress’s authority under the Declare War Clause. Where this line lies is unclear, of course, and OLC attorneys are likely to defer to policymakers on relevant factual assessments such as the risk of escalation, which limits how constraining this test may be in practice. At a minimum, however, the logic of the 2018 Syria opinion should make it difficult to justify a U.S.-initiated, large-scale ground invasion of Iran aimed at regime change, along the lines of what the United States pursued (with congressional authorization) in Iraq in 2003. And any continuing direct engagement of a sustained pitch that puts U.S. personnel at risk is likely to trigger the War Powers Resolution’s 60- to 90-day time limit, requiring the Trump administration to eventually go to Congress or articulate some reason why that limit does not apply.
Yet, as Jack Goldsmith has written for Lawfare, the Syria opinion doesn’t reflect the only logic that the OLC has applied to the use of military force. Two OLC opinions written shortly after the September 11 terrorist attacks assert a far broader view of the president’s authority to use military force. In this reading, the president’s authority is not limited by Congress’s Article I “declare War” authority or—at least in cases of self-defense from an external attack—statutory constraints like the War Power Resolution. The exception is where Congress withholds appropriations, though even then there are limits on the conditions Congress can impose. While the OLC later limited or repealed many of the opinions from this period, it left these two untouched. Goldsmith maintains that the OLC and other executive branch attorneys could cite these opinions as support for a broader vision of the president’s constitutional authority, especially when used to defend the United States from an external threat.
Moreover, even if the OLC were to apply the same logic as in its Syria opinion, it does not have the last word: Both the president and the attorney general can supersede the OLC’s legal views. Several of the president’s foreign policy advisers are lawyers who have hinted at broad views of the president’s Article II authority in the past. Attorney General William Barr has suggested that he possesses similar views—most notably in a 2000 interview in which he recalled advising then-President George H.W. Bush that he had the constitutional authority to direct the 1990 ground invasion of Iraq without congressional authorization, either directly or in anticipatory defense of the U.S. military personnel already stationed there. Adopting these views—or openly returning to post-September 11 legal thinking—is likely to prove controversial and come with political costs, particularly given that it is in tension with the Trump administration’s prior express and implied positions on the matter. But the Trump administration may determine that doing so is worth the added operational flexibility these theories permit—especially if it anticipates pursuing the sort of large-scale, long-term military operations that might run afoul of the limitations the Syria opinion seems to recognize.
Statutory Authority and the AUMF
But Trump may not need to rely on his own constitutional authority to go to war with Iran. In meetings with legislators, Trump administration officials have reportedly argued that Congress has already authorized the use of force against Iran, specifically through the 2001 AUMF currently used to authorize operations against al-Qaeda and its affiliates. Enacted just three days after the September 11 terrorist attacks, the 2001 AUMF’s operative language reads:
[T]he President is authorized to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons.
As the Obama administration described in its 2016 framework report, the executive branch has interpreted the AUMF as authorizing the use of force against al-Qaeda, as the perpetrators of the September 11 attacks; the Taliban, for having harbored al-Qaeda both before and after the attacks; “associated forces” of al-Qaeda and the Taliban, meaning organized armed groups that have “entered the fight” against the United States alongside them; and other entities, like the Islamic State, which evolved from al-Qaeda and have continued its mission. The Trump administration did not amend or object to these interpretations in its supplement to this report, but it did assert that the AUMF also authorizes the use of force to defend allies engaged in military campaigns against other valid AUMF targets from attacks by third parties, in a practice called “collective self-defense.” Thus far, the only foreign states that have been directly targeted pursuant to the AUMF include the Taliban-era Afghanistan (for harboring al-Qaeda) and, in a number of isolated incidents, the Assad regime in Syria (in collective self-defense of local militias engaged in U.S.-backed counter-Islamic State missions).
While the Trump administration has not gone public with its case as to why Iran may be targeted under the 2001 AUMF, officials have pointed to certain interactions it has allegedly had with al-Qaeda. Iranian operatives and their Hezbollah affiliates are believed to have provided occasional training and arms to members of al-Qaeda and related groups prior to the September 11 attacks, despite their deep religious and ideological differences—a claim recorded in the 9/11 Commission Report, among other official sources. And, since 2001, Iran has permitted some associates of al-Qaeda—including members of Osama bin Laden’s family—to reside and operate within its territory while detaining others. In 2011, these interactions led the State Department to accuse Iran of being unwilling to bring al-Qaeda members to justice and “allow[ing] [al-Qaeda] to operate a core facilitation pipeline through Iranian territory[,]” an assertion the department has since reiterated. That said, experts who have reviewed relevant internal al-Qaeda documents have concluded that al-Qaeda generally views Iran as hostile and has mostly operated clandestinely within the country due to distrust of Iranian officials. To the extent that Iran and al-Qaeda have collaborated, their interactions have been limited and almost entirely transactional.
Based on these facts, Iran certainly hasn’t “entered the fight” alongside al-Qaeda so as to qualify as an “associated force” under the AUMF. Nor has Iran provided al-Qaeda with the sort of refuge provided by Afghanistan during the Taliban era. And while Iran-associated forces have helped the Assad regime target U.S. counter-Islamic State allies in Syria, using this as a basis for broader military action against Iran would be a dramatic expansion of collective self-defense. Several experts have also argued that the AUMF reaches only those who “aided” and “harbored” al-Qaeda leading up to the September 11 attacks, an interpretation that would limit the relevance of any post-2001 ties and put the emphasis on conduct from almost two decades ago. Even if Iran’s pre-2001 actions check the necessary legal boxes, it seems unlikely to be a politically persuasive justification for the use of military force in 2019.
Whatever one’s preferred interpretation of the AUMF, the known facts present an incredibly slender reed on which to invoke its authority against Iran. That said, some cooperation does appear to have occurred. And the AUMF is expressly deferential to the president’s determination whether a given entity “aided” or “harbored” al-Qaeda. For this reason, executive branch lawyers seem unlikely to conclude that this argument is legally unavailable, even if they advise it is weak and likely to be controversial.
Reliance on the 2001 AUMF may, however, present operational advantages. The War Powers Resolution’s 60- to 90-day requirement for withdrawal does not apply where there is statutory authorization. Statutorily authorized uses of force also need not be concerned with limitations imposed by the Declare War Clause. And the scope of the AUMF includes “all necessary and appropriate force[,]” as informed by past practice and the international law of armed conflict. For this reason, reliance on the AUMF may be tempting if the Trump administration wishes to pursue an extended, large-scale military intervention of a scale that might trigger constitutional concerns if pursued under the president’s own authority—especially if it assesses that relying on the AUMF is more justifiable or less politically costly than asserting a broader view of the president’s Article II authority.
A final set of legal constraints on the president’s authority to use military force comes from international law. The United States’s international legal obligations apply to the country as a whole, regardless of the domestic legal basis for its actions. There is a dispute, however, as to whether international law imposes direct limits on the president’s authority by virtue of his constitutional obligation to “take Care that the Laws be faithfully executed.” While many scholars and legal practitioners maintain that it does, the OLC—most recently in a 1989 opinion by Barr—claims that it does not. Given Barr’s current role as attorney general, it seems unlikely that the current Justice Department will give Trump contrary legal advice. That said, acting consistent with international law will, at a minimum, still be important for U.S. diplomatic relationships, especially if the Trump administration hopes to secure international support for its actions.
The U.N. Charter normally prohibits states from using military force against each other absent authorization from the U.N. Security Council. But it provides an exception for states that are exercising their “inherent right of collective or individual self-defence.” The breadth of this exception is a subject of debate. The view expressed by the International Court of Justice—and largely accepted by much of the international community—is that this right is triggered only by acts (or, in some views, imminent threats) of violence that rise above a certain substantial threshold of “scale and effects[.] The United States, however, has argued that the right of self-defense applies more broadly to “any illegal use of force[,]” potentially including cyber operations and other unconventional types of attacks. It has also asserted that the concept of “imminence” should be defined by reference to a multifactor analysis that includes the nature of the attack, whether it is part of a pattern of armed activity, the likely harm it will inflict, and the probability of other opportunities to mitigate the harm—a flexible test that gives substantial discretion to U.S. political and military leaders.
As Ashley Deeks and I have written for Lawfare, to justify a military response against Iran under this standard, the United States could try to claim collective self-defense with the countries whose ships Iran has disabled in the Persian Gulf or individual self-defense in response to the recent Iranian attacks on its drones—but both may encounter objections from the international community (and even the victim states themselves) due to the limited harm inflicted by Iran’s actions. A slightly more persuasive legal basis may be the rocket attacks that Iran-associated militias have launched against U.S. military and diplomatic personnel stationed in Iraq, assuming these attacks can be credibly attributed to Iran—though they have not resulted in fatalities in several years. The United States could also attempt to make even more attenuated claims of self-defense in response to Iran’s support for terrorism or its attempts to acquire nuclear weapons, both of which would be similar to international law arguments that the OLC made prior to the 2003 U.S. invasion of Iraq—but international and domestic audiences are likely to view such claims with extreme skepticism. Alternatively, the Trump administration may simply choose to offer no international legal justification for its actions, as it did in relation to its 2017 and 2018 airstrikes on Syria.
Regardless of the justification that the United States offers, any decision to use military force would be subject to a number of international legal requirements. Most notably, any resort to military force would need to be necessary and proportional to the self-defense (or other) objectives the United States has identified in order to comply with customary international law jus ad bellum requirements. This sets some substantial limits on the scope of military activity the United States can undertake. The executive branch generally evaluates proportionality regarding what force is “required to repel [an] armed attack and ... restore the security of the party attacked[,]” which can include “force ... reasonably necessary to discourage future armed attacks or threats thereof” where a given act is “part of an ongoing pattern of attacks[.]” This most likely explains the projected casualty account of 150 people for the retaliatory airstrikes that Trump reportedly authorized and then canceled: The proposed operation would most likely have sought to substantially degrade Iran’s ability to make future surface-to-air missile attacks on foreign aircraft in the area (and, possibly, the sabotage of maritime traffic as well)—a response that the U.S. military would view as “proportionate,” even if Trump ultimately disagreed. At times, executive branch lawyers have interpreted these requirements even more permissively, to the point that they were portrayed as permitting regime change in response to threats relating to the use of weapons of mass destruction in an OLC opinion prior to the 2003 U.S. invasion of Iraq—one of the same opinions that Goldsmith addresses. Returning to similar logic would no doubt be controversial both domestically and internationally, but it’s again not legally impossible for the executive branch to pursue.
What Can Congress Do?
Thus far, many members of Congress have seemed unpersuaded by possible arguments for the president’s legal authority to use military force against Iran. While some members have argued that the president can and should respond to Iran’s provocations with military action, many others have insisted that it must seek authorization from Congress. And while Congress has not enacted any relevant legislation, it has expressed serious doubts regarding the administration’s 2001 AUMF argument—specifically, in August 2018, when it enacted a National Defense Authorization Act (NDAA) whose accompanying joint explanatory statement asserted that “the conferees are not aware of any information that would justify the use of military force against Iran under any other statutory authority[,]” a category that includes the 2001 AUMF.
The most straightforward step Congress could take if it wishes to oppose war would be to double-down on this assertion and enact legislation making clear that the 2001 AUMF does not authorize the use of force against Iran—a step that several members of Congress are actively debating. If such legislation were enacted, the president would still retain any constitutional authority he may have to use military force against Iran, including the ability to respond to attacks on U.S. drones and other Iranian provocations. But these actions would be subject to both constitutional limitations and the requirements of the War Powers Resolution, at least under the logic articulated in the OLC’s Syria opinion.
The Senate is also considering legislation that would put statutory limits on the president’s constitutional authority by expressly prohibiting and withholding funds for the use of military force against Iran. As the president’s constitutional authority to use military force is premised in substantial part on congressional acquiescence, OLC opinions cited by the 2018 Syria opinion—but not that opinion itself—describe the president’s authority to use military force as existing “at least insofar as Congress has not specifically restricted it” and accept at least some statutory limits as constitutionally valid. There are limits to how far these restrictions can go before intruding on the president’s exclusive constitutional authority as commander in chief; for example, Congress cannot dictate “operational and tactical command” decisions. And even if prohibited by statute, the executive branch is likely to assert that it has some residual exclusive constitutional authority at least to respond to imminent threats and actual attacks against the United States—an assertion arguably supported both in case law and by nonbinding provisions of the War Powers Resolution. For this reason, Congress might wish to make some statutory allowance for such actions so as to avoid inviting the executive branch to create precedents by asserting and relying on such constitutional authorities.
Even if the Trump administration ultimately advances a broad view of the president’s Article II authority that rejects the constitutional validity of these congressional limits, enacting a contrary statutory provision will still make clear Congress’s institutional opposition. This would place Trump’s authority at its “lowest ebb” under the Youngstown framework, meaning he would be able to rely only on his own exclusive constitutional authorities. This is a far higher threshold for the executive branch to satisfy, if or when its actions become subject to judicial review. And while federal courts have often resisted intervening in such cases, the Supreme Court has held that the judiciary has a “duty” to resolve cases where there is an express conflict between a statute and the president’s claimed constitutional authority. Congress could also take the novel step of authorizing litigation on its behalf, increasing the odds that a plaintiff will be found to have legal standing to bring suit—a recurring issue in prior similar litigation.
The challenge for any of these legislative fixes, ultimately, will be making it past a presidential veto. While measures limiting the president’s authority to use military force against Iran might plausibly garner majority support in the House and the Senate, they seem unlikely to secure the two-thirds support in both chambers that would be needed to override Trump’s veto. This is why existing proposals are mostly being debated within the context of the annual NDAA and defense appropriations bill, as tying such measures to high-priority omnibus legislation may make it much more politically difficult for Trump to use his veto. But there is no guarantee that these provisions will survive to the final draft of each bill. And, even if they do, the legislation currently being debated is unlikely to become law before the fall of 2019.
Hence, even if Congress chooses to act, President Trump will most likely have what his legal advisers view as legal authority to use military force against Iran for the next several months. He will have the strongest legal grounds to act if Iran is foolish enough to take action against U.S. nationals or personnel, but his legal advisers most likely do not see his authority as being strictly limited to that scenario. His administration’s own stated views also imply that there are constitutional limits on his ability to pursue large-scale, extended military campaigns involving substantial hostilities and U.S. ground forces—the type of operation that would most likely be required for effective regime change—without congressional authorization. Yet there are precedents for legal arguments the Trump administration could employ to reinterpret or avoid those limits, as long as he is willing to accept the political costs.
For these reasons, to the extent Trump feels limited in his options on how to respond to Iran, these constraints are likely to be experienced primarily as diplomatic, political and strategic in nature. This does not mean that the law is irrelevant, only that, in the absence of meaningful judicial review or any other enforcement mechanism, relevant legal arguments—and the legitimacy they provide—work through all three avenues to indirectly constrain the executive branch. This is not as satisfying as a binary view of the law that draws sharp lines around what conduct is legally impermissible and somehow enforces them against the president. But it is a more realistic account of how the separation of powers operates in practice, and it better describes the factors that will most likely determine whether and how we go to war with Iran.