Here we are again. Another presidential administration, another airstrike against another organized armed group against whom Congress has never authorized force, justified by self-defense, officially said to be necessary in light of some unstated, unspecified future threat.

The United States has engaged in a long series of one-off clashes with various militias connected to Iran, and in some cases directed force against Iran itself. Some of these have resulted in casualties. Most recently, a February 15 attack by Iranian-supported militias on U.S. and coalition personnel stationed in Iraq killed a Filipino contractor, also wounding a U.S. service member and several U.S. contractors. Last January the United States targeted and killed Iran’s senior military leader, General Qassem Soleimani, in Iraq. In its strikes, the United States has used force in or against at least four states—Iraq, Iran, Syria, and Yemen. Not once has the U.S. President sought or received congressional authorization to use force against the groups it has struck or against these states with the exception of Iraq, against whom Congress authorized force twenty years ago to address the purported threat from the long-since toppled regime of Saddam Hussein. We now appear to be sliding into war even if unintentionally, one legally-rationalized strike at a time, without real congressional debate and as a result without an opportunity for the public to grapple with the president’s reasons for doing so—or any clarity on whether such reasons in fact exist.

Some members of Congress have now proposed war powers reform measures geared toward reining in the president’s use of force untethered from Congressional approval. One such measure would repeal the 1991 and 2002 AUMFs authorizing force against Iraq, on which the last administration relied in part for the Soleimani strike. During that administration, in a bipartisan effort, Congress managed to pass a resolution directing the president to end hostilities against Iran unless explicitly authorized by Congress, a measure that President Trump vetoed. For its part, the Biden administration has expressed agreement with repealing and revising existing force authorizations. But in order to do more than pay lip service to constraining the president from taking the country to war unilaterally, we need to grapple with the legal mechanisms through which presidential administration after administration has legally justified escalating, elongating, and expanding conflicts over the last two decades.

Escalation of Conflict into War

In the first half of the 20th Century, the world essentially outlawed war. Or at least, we outlawed starting a war. In a string of agreements culminating in the U.N. Charter, the United States along with the rest of the global powers agreed to prohibit the use of force in another state’s territory, except when necessary to act in self-defense to an armed attack or acting under the authority of the UN Security Council. The differentiation in the terms “use of force” versus “armed attack” was intentional. Any use of force was prohibited. Only those uses of force rising to the level of an armed attack would permit a use of force in return.

Today, well, here we are. The United States is engaged in a series of violent conflicts across a range of states. The U.S. military is prosecuting wars with groups most Americans have never heard of. And despite the constitutional delegation of war-declaring authority to Congress (along with a host of other war-related powers), the United States is at war with groups on which Congress has never passed judgment.

How did this come to pass, how does this continue to happen? Some take from this state of affairs that U.S. Presidents simply care not one whit about following the law, whether it be the UN Charter or the U.S. Constitution. Yet decisions the executive branch makes in war are heavily lawyered. The executive branch may not release a legal memo every time it drops a bomb, but each of these decisions rests on the justifications developed by executive branch lawyers analyzing statutes, treaties and customary international law, internal regulations, the occasional Supreme Court decision, the occasional International Court of Justice decision, and, most of all, prior executive branch practice and prior executive branch memoranda.

At many of the incremental steps that got us to where we are today, executive branch lawyers have relied and built upon executive branch precedent to justify their actions, both as a matter of international and domestic law. In terms of domestic authority, government lawyers bless the president’s unilateral uses of force against states and non-state actors as long as they conclude the actions satisfy some self-proclaimed “national interest” and fall below a conceptual threshold of not-actually-war, defined by decades of government lawyer memoranda that have crafted a high bar for “war in the constitutional sense.”  And on the international law side, government lawyers have sanctioned force if it meets their fairly low bar for a finding of necessary “self-defense.” But the result over the long haul is war. War without that one unmistakable act of unilateral aggression. War slowly backed into over time, largely outside the public gaze, legally justified step by legally justified step. War, once begun, expanded to ever new groups. This result—an ever-expanding forever war—is hardly unforeseeable. And the legal moves, while incremental, are often in full view. We just need to know what we are looking at. And there are some clear hallmarks associated with the set of legal moves to which we can be alert in real time.

Unilateral acts of aggression are prohibited under international law. Self-defense, of course, is an exception to the prohibition of the use of inter-state force. One way that states slide into war without acknowledging an act of aggression is to claim their acts fall within the self-defense exception. And many states, including but not limited to the United States, have promoted a fair amount of interpretive space in these rules. Any use of force, according to US government lawyers, at least by a state actor, can trigger the right of self-defense; in the U.S. view, there is no minimum threshold a use of force must meet in order to qualify as an “armed attack,” though there are legal limitations on the force that can be used in response, which must be “necessary” and “proportionate.” Moreover, the United States claims the right to respond with force not just to actual armed attacks but to “imminent” armed attacks. It has extended this claim beyond specific imminent attacks it knows will happen to more indeterminate future threats as well. And it claims a broad remit when considering imminent threats by non-state actors acting within states that may themselves have shown no hostility toward the United States. This much is well covered territory.

There is another legal move in the United States’ toolkit that is less scrutinized, and this is the justification of unit self-defense. Unit self-defense is defense not of the state itself, but of the state’s armed forces. And it is a significant mechanism for escalation of conflict, whether intentional or not. Consider how these legal plays work in context. Under the U.S. military’s Standing Rules of Engagement (SROE), issued by the Joint Chiefs, “Unit commanders always retain the inherent right and obligation to exercise unit self-defense in response to a hostile act or demonstrated hostile intent.” These rules apply to U.S. armed forces that may be deployed abroad to contexts where their very presence may quite foreseeably inflame tensions.

One state may consent to the presence of U.S. troops, providing the legal justification for that move. But once there, a threat to these troops provides the United States with significant discretion to escalate the situation, under its unit self-defense theory. Under the SROE, a mere “hostile act or demonstrated hostile intent” could provide sufficient basis to respond with force. The result may be the United States using force against non-state actors who do not even have the capacity to threaten U.S. territory, in a state that has not attacked the United States, providing the groundwork for a future escalation with either that non-state actor or the state itself—and all without authorization from Congress or any opportunity for the American public to consider and debate whether this is a war they wish to initiate or otherwise become embroiled in. Moreover, the executive branch has further taken the position that these theories of self-defense extend to partner armed forces, including non-state partners, under a broad conception of “collective self-defense.” As I discuss below, in the case study on al Shabab, this can result in the United States using force, without congressional authorization, against a novel group that has never attacked us, in a state that has never attacked us, in defense of a perceived threat to another state’s or group’s forces. As a result of concerns about this latter slippery slope, Congress included in the 2020 NDAA a requirement that the Secretary of Defense provide a report on the use of U.S. forces in “collective self-defense” of foreign nationals or property.

Many of these claimed legal authorities tend to fly under the political radar but they do real work in the escalating conflicts in which the United States continues to engage. These situations raise significant questions of policy as much as—or even more than—law. But understanding that the United States claims such legal authority—and the significant slope these legal moves create to slide into escalatory conflict—should inform policy determinations about deploying troops abroad from the beginning.

Members of Congress, the courts, and the public all accept some level of unilateral action on the part of the president as long as it feels incremental, and as long as there is a sense of limiting principles. But these authorities and the examples I discuss below demonstrate how pliable those limiting principles are, and how easily those incremental steps can ratchet toward full blown war. With this in mind, we should not be overly mollified by assurances of narrowly cabined authority or “limited strikes” and instead consider, at the outset, where all of this is heading as a matter of policy.

Expansion of War

As readers of Just Security are surely aware, the executive branch has over the two decades since 9/11 expanded its conception of the conflict with al Qaeda through creative legal interpretation of both the international and domestic law architecture related to it. Much of this expansion has come through unilateral steps by the executive branch—typically through legal interpretation followed by kinetic action, or the other way around per the al Shabab example below—with some acquiescence or constructive ignorance from the other branches.

The most notable of these moves is the concept of “associated forces,” a term the executive branch crafted to identify groups that are connected to al Qaeda and meet relatively loose criteria. By relying on this concept and suggesting (through terms like “co-belligerency”) that it has some clear limiting principle derived from international law (it does not), the executive branch has successfully asserted that its domestic statutory authority to use force extends beyond the core groups referenced in the 2001 AUMF (those who were responsible for the attacks of 9/11, a position that has been ratified over time by the courts and Congress (although the specific groups have not been). The executive branch has extended this theory further with the concept of “successor” forces—groups that may have past ties to al Qaeda but who may no longer share its mission, even groups long disassociated from or in an open rift with al Qaeda—against whom the executive branch uses force under claimed congressional authority through the 2001 AUMF. This is the government’s domestic legal theory for the ongoing conflict with ISIS, a group that did not exist in 2001.

This position means that the executive branch, through its own legal interpretation of a 20-year-old authorization—a statute that was intended to sanction an immediate response to the 9/11 attacks—and its application of that legal theory to facts it alone may access, may continuously update its authority to use force against new groups without going back to Congress. By doing this through legal interpretation, the executive branch gives all three branches—the executive, the courts, and Congress—some political cover in avoiding responsibility for the president’s policy decision to expand the conflict. Policy makers can point to lawyers, lawyers can point to their legal jargon. The result is that the president holds the unilateral power to extend the war to groups that many Americans do not know exist.

Any efforts toward new or revised AUMFs must account for how the executive branch has interpreted past authorities and include clear language regarding targeted groups, strict parameters, and rules for how the force authorization will sunset.

Expansion of the Battlefield

The executive branch has also extended the geography of conflict through legal interpretation. The UN Charter’s prohibition against the use of force is a protection of the territorial state in which that force is used. The exception enshrined in Article 51—preserving states’ right to self-defense “if an armed attack occurs against a Member of the United Nations”—does not specify whether a state must be the perpetrator of the armed attack. If a state is subject to an attack from a non-state actor, operating from another state’s territory, may the victim state respond with force in the territorial state without the latter’s consent? This question has long been a point of controversy, but the United States has taken the view that it may lawfully use force even in a territorial state as a matter of necessity if that state is unwilling or unable to eliminate the threat itself.

On that question alone, whether states can use force against non-state actors in a non-consenting state, several other states have now accepted the general unwilling or unable construct—though other states have opposed it. The U.N. Security Council itself affirmed the right of states to respond in self-defense in the aftermath of the 9/11 attacks, suggesting implicitly that a state might be subject to an armed attack by a non-state actor such as al Qaeda. Of course, the breadth of the unwilling or unable test must be understood in light of the United States’ other legal positions. Under U.S. interpretations, the test might permit the United States to attack not just in response to an actual armed attack, but potentially in response to a threat. And some U.S. statements suggest a possibly lower standard still—that once the United States has made a determination that it may attack a specific armed group somewhere in the world, it may attack targets connected to that group anywhere, regardless of whether an imminent attack is actually emanating from a particular territorial state on which it wishes to use force, as long as that state is unwilling or unable to keep the group from using its territory at all. This is a question on which further clarity from the administration would be useful. It is open to question whether the rule would be limited to situations in which the non-state actor “is using the State’s territory as a base for attacks and related operations” (per a 2016 government transparency report), and what limits the term “related operations” will hold among U.S. government lawyers.

By finding a state “unwilling or unable” to mitigate a threat posed by actors, the United States has surmounted sovereignty hurdles that would otherwise prohibit geographic extension of a conflict to a state that had not attacked us. And for decisionmakers facing a complex policy call—whether to use force in a new state’s territory—it provides a handy test: is the territorial state unable or unwilling? Substituting a legal test for a policy decision need not be the result—surely policymakers need not take all actions that their lawyers deem legally available. And certainly, in many cases policy makers may want to take a particular action and then push for an aggressive legal interpretation to justify it. But for policymakers seeking both to take aggressive action and to provide an assurance of limiting principles on that action, they may find something legitimizing, both for their own needs and for others, in a legal test. For members of Congress this substitution may be even starker: the assurance that the executive branch is exercising power within limits makes it easier for them to fail to probe too hard into whether and why the president’s use of that power is wise. This is all the easier if government lawyers can muster a plausible justification for why the president may act under domestic legal authorities he already holds. The legal architecture can shortchange the policy debates and avoid some of the messiness, and the responsibility, that a public debate about using force against a sovereign state’s territory would generate.

The Al Shabab Effect

The United States’ slowly escalating conflict with al Shabab over the course of a decade in the “war on terror” is a useful case study of several of the legal moves described here, and a cautionary tale about how this incremental lawyering and kinetic action can ultimately result in backing into war.

Al Shabab is an armed group that has long sought control in Somalia. Certain members of the group had ties to al Qaeada, and for years, executive branch officials reportedly debated whether to classify al Shabab as a whole as an associated force of al Qaeda. Doing so would have brought the group within the ambit of the ever-expanding armed conflict, including far flung members who had never sworn their allegiance to al Qaeda and may not even have known about their leaders’ connections to the group. Instead of taking that approach, however, government officials compromised among themselves, classifying as al Qaeda or associated forces only those high-level al Shabab leaders who themselves could be considered functionally akin to members of AQ.

This legal distinction meant that the United States was not technically at war with al Shabab. In practice, the most important consequence was that the United States could not target or detain a run-of-the-mill al Shabab foot soldier. And yet, in the spring of 2016, the press suddenly began reporting U.S. air strikes against al Shabab troops. DOD soon released a statement in a response to New York Times reporter Charlie Savage that the U.S. government had not suddenly changed its classification of al Shabab. Instead, U.S. forces had taken the strikes in the name of unit self-defense—of both U.S. and AMISOM forces. Had those forces actually come under attack by al Shabab? Not exactly. Instead, the U.S. position was that al Shabab fighters posed an “imminent” threat, justifying the strikes in “tactical” defense of US and AMISON forces. In short, despite debates raging for years inside the government over the extent to which the U.S. government could lawfully use force against al Shabab under one legal theory, the U.S. government subsequently used force anyway under a different legal theory.

That incremental approach itself brought together several legal theories that are in themselves somewhat complicated, with room for lots of gray—unit self-defense, defense of other “allied” forces, imminent strikes—each of which moves the ball forward a tiny step. When taken together these combined steps led to U.S. forces taking actions against another entity that would seem to bring the two very close to full-blown armed conflict, whether or not the U.S. government strictly classified it as such.

A few months after these strikes, in the twilight of the Obama administration, the U.S. government finally classified al Shabab as an associated force in a December 2016 war powers report to Congress, thus completing the escalation. Al Shabab would now be considered a full party to the conflict with al Qaeda. Given that at that point the U.S. military had already undertaken several strikes against al Shabab troops, there may have been fewer internal hurdles standing in the way of such classification. Some have noted that there may have been new intelligence changing the calculus; others have noted that there is always new intelligence and there may have simply been incentives to view that intelligence in a different light. In any event, many of the internal players—including the loudest dissenting voices—had by that time moved on. It is very possible that internal dissent simply lost steam once the practical significance of classification slowly disintegrated. But the legal significance of this status change should not be dismissed. Classifying al Shabab as an associated force meant that we were at war. It was a claim to authority to target and kill any member of the group, no matter what actual threat they posed. And it was an assertion that Congress had already authorized force against al Shabab in the 2001 AUMF and would not need to pass further judgment on the wisdom of going to war.


A new presidential administration is an opportunity for reflection, for those inside and outside the administration. For twenty years, three presidential administrations and the bureaucratic architecture of the U.S. executive branch have embroiled us further and further in conflicts across varied groups and continents, killing combatants and civilians alike, inflaming tensions across the world, and much of this flying almost entirely under the radar of the American public. The international and domestic law here are deeply intertwined, and both are doing some work in pacifying concerns and justifying continued war. These legal moves are not the cause of the continued conflict, but they are surely not hindering it.

This is not to say that law serves no purpose in war; I very much oppose that view and it is belied by the evidence. Law, as I describe it here, is often a tool, yes. But it also keeps the conversation within one sphere of behavior with some agreed upon ground principles and not, at least, on the other side of the room. And considering the historic carelessness of those in power with individual lives, it can and should provide an absolute floor for the treatment of individuals. This is particularly critical for jus in bello, and other laws governing protections afforded individuals within conflict. As a practical matter, the black letter law governing jus ad bellum is so much sparser, and state practice more varied, that law may reasonably seem more malleable in the context of macro decisions of when and whether to go to war. As a result, it is in particular on the domestic procedural question of who within the United States makes that decision, the president or Congress, that the promise of international law as a limiting principle may obscure the president’s power aggrandizement more than it constrains.

The primary problem here, however, is not that these legal arguments are beyond the pale, though some of them may be. Nor is the problem that the executive branch is acting without concern for the law; the intense internal legal debates at a minimum surely suggest otherwise. As do the countless lawyers throughout the government who devote their time and, in many cases, entire careers to public service. The problem is not even that the executive branch’s legal positions are insufficiently constraining as law, through in many cases they are not. But the focus on legal justification, and legal justification done incrementally in particular, obscures the reality that we are sliding into or expanding war. Satisfying these legal tests is an insufficient replacement for the exercise of policy judgment. Each raises distinct questions, though there are overlaps. (After all, law is often the product of decisions to enshrine areas of policy agreement.) Policy makers want to project strength but with limiting principles. And legal argumentation provides a suggestion of rigor, of limitation, and of legitimacy that satisfies some policy makers who are looking for these features.

Most of all, this focus on law has allowed the public and members of Congress to look the other way, to allow the president to act unilaterally under the guise that as long as he can explain why his actions are lawful, they are limited and legitimate. Understanding the elasticity of these legal concepts, the weaknesses in these limiting principles, and the ways in which international law is used to expand the president’s power as much as it is a constraint exposes that legal justification is not doing all of the work assumed of it. If nothing else, the above discussion demonstrates that law will not keep us from sliding into war. It is therefore well past time to start bringing these questions to the public from the outset, before it is too late.

IMAGE:  Yemeni child on March 18, 2018, looking out at buildings that were damaged in an air strike in the southern Yemeni city of Taez. (Photo by AHMAD AL-BASHA/AFP via Getty Images)