This past Sunday, Dec. 29, U.S. military forces conducted airstrikes against five facilities in Iraq and Syria controlled by Kata’ib Hezbollah, an influential Iraqi militia with close ties to Iran and an official status as part of Iraq’s security forces. The Defense Department later confirmed that the airstrikes—which reportedly resulted in more than 25 fatalities—were in response to a series of rocket attacks against various facilities housing U.S. personnel. Their stated objective was to “degrade [Kata’ib Hezbollah]’s ability to conduct future attacks against [U.S. and allied] forces” by eliminating “weapon storage facilities and command and control locations … use[d] to plan and execute attacks”—including the most recent Dec. 27 attack outside the city of Kirkuk, which wounded several U.S. and Iraqi military personnel and killed a U.S. citizen military contractor.

Several U.S. political leaders have applauded the Trump administration’s actions as a necessary and appropriate response to Iranian aggression. They have also tried to frame U.S. actions as pushing back against undue Iranian interference in Iraq’s internal affairs. Individuals across Iraq’s political spectrum, however, have condemned the airstrikes as a grave violation of Iraqi sovereignty, even while criticizing Kata’ib Hezbollah and Iran. One of the most important such statements came from the influential religious figure Ayatollah Ali al-Sistani, who asserted that “illegal practices carried out by some sides must not be used as a reason to violate Iraq’s sovereignty” and called on the government to “to ensure Iraq does not become a field for settling regional and international scores.” The office of beleaguered Iraqi Prime Minister Adel Abdul-Mahdi followed suit by describing the airstrikes as a “vicious assault” that would have “dangerous consequences.” As the airstrikes were “a grave violation of the rules of action of the coalition forces” present in Iraq, Abdul-Mahdi later announced, the Iraqi government intended to “review the relationship” so as to “preserve[] the sovereignty and security of the country”—spelling potential trouble for the U.S. military presence in Iraq.

Kata’ib Hezbollah, meanwhile, promised retribution, even as it and Iran have continued to deny any involvement in the Dec. 27 rocket attack. On Tuesday, Dec. 31, hundreds of supporters of Kata’ib Hezbollah who had been attending a funeral service for those killed in the airstrikes marched into Baghdad’s secure Green Zone and breached the walls of the U.S. Embassy in Baghdad, Iraq, setting fires and forcing U.S. diplomatic staff to retreat before Iraqi security forces could intervene. Among the group were several members of Abdul-Mahdi’s government associated with the Hashd al-Shaabi, or popular mobilization forces (PMFs)—a contingent of sectarian militias, including Kata’ib Hezbollah, that is formally part of the Iraqi security apparatus but often continues to operate independently.

While the demonstrators originally indicated that they intended to stay until the embassy shut down, Abdul-Mahdi has since ordered that they disperse and even threatened them with arrest. Kata’ib Hezbollah’s leadership later agreed to comply with this request and the demonstrators withdrew. Several political allies of President Trump have in turn already blamed Iran for the embassy incursion and urged the Trump administration to pursue further measures to hold it accountable.

The U.S. decision to directly target Iranian affiliates within Iraq without the permission of Congress or the Iraqi government raises a number of difficult legal and policy questions. As a legal matter, the airstrikes are consistent with measures the United States claims the legal authority to pursue in defense of its personnel, under both domestic and international law. Yet this conclusion relies on certain idiosyncratic interpretations of international law and related facts that Iraqis are almost certain to reject, contributing to the view that U.S. actions violated Iraq’s sovereignty.

As a policy matter, the resulting U.S. overreach is likely how Iran and its allies hoped the United States would respond to their provocations. By stepping into this familiar trap, the Trump administration has once again made the United States the focus of Iraqis’ ire, relieving some of the pressure on Iran that has been building over several months of popular protests in Iraq. The results are unlikely to fundamentally change the broader regional struggle between the United States and Iran. But they may have lasting implications for the increasingly tenuous relationship between Iraq and the United States, whose future is now more uncertain than ever.

Putting the Strikes in Context

The weekend’s exchange of hostilities is the latest chapter in a particularly tense period in U.S.-Iran relations. Over the past two years, the Trump administration’s decision to withdraw from the Iran nuclear deal and replace it with a “maximum pressure” sanctions campaign has pushed Tehran toward unprecedented levels of brinkmanship. In 2019, attacks by Iran and its proxies on foreign oil tankers, a U.S. drone and Saudi oil facilities almost triggered a U.S. military response that was ultimately aborted by Trump. The substantial U.S. diplomatic and military presence in Iraq, meanwhile, has continued to provide Iran with a valuable source of leverage—one Tehran has capitalized upon by escalating rocket attacks and other threats against U.S. personnel.

These regional tensions have coincided with a domestic crisis within Iraq. Since the end of the Iraqi military campaign against the Islamic State, the PMFs—including Kata’ib Hezbollah—have become increasingly powerful. Both Abdul-Mahdi and his predecessor have repeatedly tried to secure control over the PMFs by formally incorporating them into the state’s security apparatus, but these efforts have largely failed. Quite to the contrary, a coalition of Shiite PMFs with ties to Iran’s Quds Force came in second place in Iraq’s 2018 parliamentary elections, giving them substantial influence within Abdul-Mahdi’s government. As a result, several of these groups—which have long histories of illegal and violent activities, often at cross-purposes to the official policies of the Iraqi government—have been freer than ever to operate in Iraq, further amplifying Iranian influence.

In recent months, however, this influence has been challenged by an unlikely source: sustained cross-sectarian populist protests targeting the political status quo, which protesters see as compromised by widespread corruption and undue foreign influence from both the United States and Iran. Iranian agents and proxies like Kata’ib Hezbollah have worked with some Iraqi officials to violently suppress these protests and preserve the current government elite but have not yet succeeded in stopping them. On Nov. 29, Abdul-Mahdi resigned in an effort to satiate the protesters, though he has stayed on in a caretaker role until Iraq’s parliament can form a new government. Nonetheless, the protests have continued.

From a security standpoint, the United States has responded to these developments with efforts aimed at both risk reduction and deterrence. The State Department has dramatically reduced the U.S. diplomatic presence in Iraq, in substantial part to limit U.S. vulnerabilities to attack. At the same time, the United States has expanded its military presence in the broader region in order to make clear its ability to respond to Iranian aggression with overwhelming force. Specifically, in regard to Iraq, Secretary of State Mike Pompeo has repeatedly warned that any harm to U.S. personnel by Iran or its proxies would be met with a “decisive response.”

Until the Dec. 27 attack, no group dared to cross this red line. When Kata’ib Hezbollah finally did so, the Trump administration no doubt felt the need to respond or else risk the credibility on which its other deterrent efforts relied.

Legal Arguments and Justifications

The Trump administration made the decision to pursue the Dec. 29 airstrikes unilaterally, without authorization from Congress or the Iraqi government. When the U.S. informed Iraqi officials of its intent to move forward with airstrikes shortly before they occurred, Iraq reportedly objected and asked the United States not to proceed, warning of grave consequences for U.S. interests in the region. Nonetheless, the Trump administration elected to proceed on its own. This is in spite of the fact that Iraqi consent is a legal precondition for other U.S. activities in the region, including the ongoing U.S. military presence within the country and—to a lesser degree—counter-Islamic State operations in Syria.

While the administration has not yet provided a clear statement of the legal theory under which it acted, remarks by senior officials and past U.S. practice provide some guidance on the logic U.S. officials most likely employed. In several regards, the Dec. 29 airstrikes appear to be largely consistent with the types of military action that the United States has claimed the legal authority to pursue in the past, under both domestic and international law. Yet this conclusion relies on several idiosyncratic U.S. legal positions that are not widely accepted—as well as a determination that the Iraqi government is unable or unwilling to address the threat from Kata’ib Hezbollah, which many Iraqis are likely to disagree with.

In regard to U.S. domestic law, executive branch lawyers almost certainly believe that the president had the constitutional authority to undertake the Dec. 29 airstrikes without express congressional authorization. The executive branch has long maintained that Article II of the Constitution empowers the president to use military force overseas in pursuit of an important national interest so long as it is of limited nature, scope and duration. While many observers disagree with these views, both the federal courts and Congress have thus far left them undisturbed. The Dec. 29 airstrikes, in turn, fit comfortably with prior practice under this legal theory: The executive branch understands retaliation against attacks on U.S. soldiers as valid grounds for using military force, and the airstrikes in question were more constrained than other operations that presidents have pursued under Article II.

Yet it appears that the Trump administration has elected not to rely on this legal theory. The 1973 War Powers Resolution obligates the executive branch to provide a report to Congress identifying the legal basis for any non-statutorily authorized military action within 48 hours, the deadline for which passed on Dec. 31. While not strictly required, such reports have generally been posted on the White House’s webpage. Yet no such report has been published to date, nor have there been reports that Congress has received one. If this situation holds, then the most likely explanation is that the Trump administration relied on statutory grounds to justify the Dec. 29 airstrikes.

The most likely candidates for such statutory authorizations are the two Authorizations for Use of Military Force (AUMFs) currently at play in Iraq. The U.S. troops currently engaged in counter-Islamic State operations in Iraq are there pursuant to the 2001 AUMF, which the executive branch interprets as authorizing the use of force against al-Qaeda and related groups, as well as the 2002 AUMF, which addresses “the continuing threat posed by Iraq[.]” The Trump administration has previously refused to rule out the possibility that the 2001 AUMF could authorize direction action against Iran and its allies. That said, such an argument is extremely dubious and is likely to prove extremely controversial. A more plausible candidate might be the 2002 AUMF, which provided the domestic legal basis for various U.S. military activities in Iraq from 2003 through 2011, including earlier operations against Kata’ib Hezbollah. Yet it would be novel, to say the least, to read a statute that has traditionally been interpreted as authorizing activities to support the democratic government of Iraq as permitting military action against an official—if largely autonomous—part of Iraq’s official security forces.

In congressional correspondence earlier this year, however, the State Department posited an alternative usage of both AUMFs: specifically, that they authorize military action in defense of U.S. and partner forces who are engaged in activities that the AUMFs authorize from attacks by third parties not otherwise covered by the statutes. The Trump administration has already deployed this logic in Syria, where it’s repeatedly taken military action to defend its local partners in the counter-Islamic State campaign from attacks by the Assad regime and its allies. During a Dec. 30 briefing, an unidentified State Department official suggested that the Trump administration might be using similar logic to justify the Dec. 29 airstrikes.

Notably, these are narrower grounds for using military force than concluding that an AUMF authorizes direct military force against a target. As the Defense Department has described in separate correspondence, such “[s]elf-defense is not a deliberate, offensive use of force ... [but] a reaction to an attack or imminent threat of attack.” Hence, concluding that Iran or Kata’ib Hezbollah could be targeted under this theory would not necessarily mean that either entity could be the subject of a broader, U.S.-initiated military campaign, as entities directly covered by the AUMFs could be. Regardless, the exact domestic legal grounds being relied on by the Trump administration are worth clarifying and would warrant further inquiry by Congress and the media.

When it comes to international law, meanwhile, the airstrikes raise even more significant questions. Generally, international law prohibits states from using force on another state’s territory without its consent. The Defense Department has indicated that the United States was “exercising its right of self-defense” in pursuing the airstrikes—a recognized exception to this prohibition, which states can exercise either individually in response to an armed attack or collectively upon another state’s request for assistance. Many are likely to dispute whether the Dec. 27 rocket attack and other prior incidents meet the threshold of “the most grave forms of the use of force” as is necessary to trigger the right to self-defense under the logic of the International Court of Justice (ICJ)’s Nicaragua decision. Yet the United States has long maintained that international law in fact permits self-defense against any deliberate use of “deadly force” by a state or armed group, even where that force does not result in fatalities—a standard that the Dec. 27 rocket attack seems to readily satisfy.

Even more controversial, however, is the proposition that the right to self-defense allows a state to intervene and use military force against nonstate actors within another state’s territory yet without that host state’s consent. The United States has argued that such action is permissible where a host state is “unable or unwilling” to address the threat posed to an intervening state by the nonstate actors, a proposition that has garnered some limited international support. This is almost certainly the international legal theory that the United States applied to the Kata’ib Hezbollah sites in Syria, as it has done for the Islamic State and other nonstate actors it has targeted there. Reaching the same conclusion in regard to Iraq, however, would be a major development. In effect, this would be calling into question the Iraqi government’s capability or intent to control Kata’ib Hezbollah and other PMFs—a view likely to be controversial with Iraqis. That said, it’s plausible that the United States would reach this conclusion, particularly given the Iraqi leadership’s inability to stop prior rocket attacks against U.S. personnel and difficulty exercising control over the PMFs in other domains.

International law also requires that any military response be necessary and proportionate. The Dec. 29 airstrikes have come under criticism in both respects, as the types of facilities targeted and the number of individuals killed far exceeds those affected by the Dec. 27 rocket attack. Yet the United States has long maintained specific views of both requirements that do not rely on such direct comparisons.

In regard to necessity, the ICJ has suggested that a state should seek alternative remedies before resorting to the use of force, including by complaining about problematic conduct to relevant foreign officials. The United States, however, has objected to this standard and maintains that international law requires only that there be “no reasonable alternative means of redress.” Here, the fact that the United States repeatedly complained to Iraqi officials about ongoing rocket attacks against U.S. facilities over an extended period of time without successful resolution may give it a reasonable argument for necessity under either standard. That said, the former may hinge on the extent to which the United States complained and sought alternate remedies in regard to the specific activities and facilities it ultimately targeted, a set of facts currently not known.

As for proportionality, the ICJ has previously examined “the scale of the whole operation” to that of the armed attack it is responding to in determining proportionality—a standard that might put the proportionality of the Dec. 29 airstrikes into serious question. Trump himself seemed to use the same standard when declaring that he had abandoned an airstrike against Iran for shooting down an unmanned U.S. drone as “not proportionate” over the summer, as it would have resulted in multiple fatalities. The United States, however, has long maintained that proportionality should instead be evaluated in relation to what is required to address the underlying threat, including the attacker’s ability to pursue future similar attacks. Hence, if the Defense Department’s statement that the Dec. 29 airstrikes were intended to “degrade [the] ability to conduct future attacks” is accurate, this would again comport with U.S. views on proportionality—even if it may not satisfy the higher threshold applied by the ICJ and other members of the international community.

Given the above, it is not entirely surprising that executive branch lawyers concluded that the president had the legal authority to pursue the Dec. 29 airstrikes without further congressional authorization or even the consent of the Iraqi government. But that does not mean that doing so was wise or that others in the international community would agree. This is especially true among Iraqis, many of whom have been deeply suspicious of the United States since its 2003 invasion and subsequent occupation of Iraq. Even U.S. military action universally seen as legal and legitimate is likely to have a cool reception in Iraq. Hence, it should have come as no surprise to the Trump administration that many Iraqis see airstrikes premised on such idiosyncratic legal grounds as acts hostile to Iraqi sovereignty. This is the risk the Trump administration assumed in choosing to act as it did—and now, the United States is living through the consequences.

Consequences and Implications

No one should doubt that the Dec. 27 rocket attack placed the Trump administration in an extremely difficult position. For months, Kata’ib Hezbollah and other groups had been ratcheting up attacks on facilities housing U.S. personnel, putting them in increased danger. Efforts to work with the Iraqi government to address these threats had seemingly been unsuccessful. Now these attacks had turned fatal, despite Pompeo’s repeated assertions that such an outcome was an American red line. The Trump administration’s entire regional military strategy for containing Iran is premised on deterrence, backed by the threat of overwhelming U.S. military force—a threat whose credibility may be compromised by failing to back up Pompeo’s earlier words with action. Given these circumstances, a calibrated military response—forceful enough to impose real costs while specific enough to deter future rocket attacks without inviting escalation in other areas—may well have seemed like the best of the bad options available, especially as the domestic political pressure to respond continued to build.

Yet this is exactly the situation in which Iran and its proxies like Kata’ib Hezbollah hoped to place the United States. These groups often employ tactics reminiscent of guerilla warfare, focusing on small, targeted actions that harass and hinder the U.S. diplomatic and military presence while capitalizing on their greater mobility and freedom to operate. Where these groups finally push the United States hard enough that it feels compelled to respond, their hope is that it overreaches in a way that triggers Iraqi distrust of the American military and further undermines Iraqi political support for the U.S. troop presence there. That is exactly what the Trump administration did with its Dec. 29 airstrikes—and it couldn’t have come at a better time for Iran.

For months prior to the airstrikes, Kata’eb Hezbollah and other PMFs had been the subject of heated criticism by Iraqi protesters for facilitating corruption and selling out to Iranian interests. The PMFs’ role in trying to violently suppress the protesters only added fuel to this fire. For its part, the United States had done a reasonably good job voicing calibrated support for the protesters and had even gone as far as to put sanctions on some of the PMF leaders involved in the violence. One can imagine a counterfactual in which the United States used the Dec. 27 attack as grounds for renewing demands that the central government of Iraq rein in the PMFs, amplifying the protestors’ own demands. This might not have yielded immediate results, but it would have allowed the United States to put increased public pressure on Iran and position itself alongside what may be the next generation of Iraqi leadership. Even if the United States had ultimately felt the need to pursue military action, it could have spent this time making its case domestically, cultivating support among other Iraqi critics of Iranian influence, and ultimately making clear to the Iraqi public that it was acting on its own only as an absolute last resort. Instead, the United States only succeeded at shifting the Iraqi public’s ire against foreign influence from Iran back to itself, giving its rival much-needed breathing space.

The most dangerous possible outcome from these events would be a sudden escalation in hostilities between the United States and Iran. While Trump and his allies have expressly blamed Iran for both the Dec. 27 rocket attack and the Dec. 31 incursion into the U.S. Embassy, this nonetheless seems unlikely. In their past interactions, both the United States and Iran have been intent on avoiding escalation into open conflict. Instead, Iran has pursued most of its provocations through regional proxies; and the United States, despite the Trump administration’s bellicose rhetoric, has generally pursued measured responses—approaches intended to hedge against unintended escalation. Iran, in particular, must be aware that it would be at a severe disadvantage in an open conflict with the United States and thus seems more likely to continue relying on tactics that are not as vulnerable to direct military response. Hence, barring a decision by the United States to initiate a broader conflict with Iran—a step that would be immensely controversial and pose major legal problems—further escalation in the U.S.-Iran conflict is likely to continue coming in further small steps.

The most direct effect of the U.S. airstrikes is likely to be on the beleaguered security relationship between Iraq and the United States. Since the 2003 U.S. invasion and occupation of Iraq, many Iraqis have viewed U.S. military involvement in Iraq with immense skepticism—sentiments that Iran and other actors have often capitalized on to undermine support for U.S.-Iraq cooperation. Nonetheless, since 2014, Iraqi leaders have consented to a limited U.S. military presence in the country to help train and advise Iraqi security forces and assist in operations against the Islamic State. While no U.S. forces based in Iraq appear to have played a role in the Dec. 29 airstrikes, the broader U.S. willingness to engage in what most Iraqis see as a violation of Iraqi sovereignty plays into popular concerns about U.S. intentions. The end result may be that Iraq’s leadership—or its parliament, which has frequently considered such measures in the past—decides to rescind or narrow its consent to a U.S. military presence in Iraq. If this were to occur, then U.S. troops would almost certainly have to withdraw to whatever levels the Iraqi government was willing to continue authorizing. This would not only undermine efforts to improve Iraq’s own security forces and prevent a resurgence of the Islamic State but also further hinder U.S. counter-Islamic State operations in Syria, which have been increasingly based out of Iraq since the Trump administration’s partial withdrawal from northern Syria in 2019.

Of course, this military relationship may yet survive the present crisis. Much of the activities it supports are to the benefit of Iraq’s own security forces, and Iraqi leaders have at various times seemed to appreciate the fact that U.S. support provides a counterbalance to Iranian influence, giving them more political space in dealing with Iranian demands. Moreover, even if it were to come about, the withdrawal of U.S. troops would not be the end of the U.S.-Iraq relationship. Both Iraq and the United States continue to share common interests in other areas, including in Iraq’s continued political and economic stability. And the United States has other sources of leverage over Iraq, most notably the important waiver to Iran-related oil sanctions that the United States generally provides to Iraq every year. That said, building and maintaining such a relationship would require a diplomatic presence robust enough to make up for the precipitous decline in military-to-military engagements. And given the Trump administration’s dramatic reduction in staffing at the U.S. Embassy in Baghdad, it’s not clear whether it is willing to commit the resources and assume the risks necessary to pursue this sort of relationship.

Indeed, the most significant outcome of the present crisis may well be that the Trump administration decides that the U.S.-Iraq relationship is simply not worth maintaining. On this logic, the diplomatic and military presence in Iraq only gives Iran a source of leverage over the United States, undermining the administration’s top priority of putting pressure on Iran. Sacrificing the U.S.-Iraq relationship would risk serious harm to various long-standing U.S. interests in the Middle East, but it would arguably allow the United States to act with a far freer hand in targeting and pressuring Iran. Given the Trump administration’s myopic focus on Iran, this may—unfortunately—be a trade-off that some officials are willing to make.