(Editor’s Note: This is the second installment of our Ending Perpetual War Symposium. The article derives from a chapter in Brianna Rosen, ed., Perpetual War and International Law: Legacies of the War on Terror (Oxford University Press forthcoming 2024) and draws upon Oona A. Hathaway, How the Erosion of U.S. War Powers Constraints Has Undermined International Law Constraints on the Use of Force, 14 Harv. Nat’l Sec. J. (2023).)

When the United States went to war in the wake of the terrorist attacks of September 11, 2001, few imagined that we would still be at war more than two decades later—and, indeed, that the United States would have ongoing military operations not only in Afghanistan but throughout the Middle East and much of Africa. While there is a great deal of blame to go around for this state of affairs, at least some of that blame rests with the lawyers, who have produced legal interpretations that have enabled war to continue and expand far beyond what Congress initially intended.

This article will focus on twin sets of legal interpretations that have together been critical to creating and sustaining the perpetual war: the erosion of domestic constraints on the president’s unilateral war powers and the ever-expanding reliance on the international law right of “self-defense.” Together, these shifting legal interpretations have reduced legal constraints on the president’s use of military force. At the same time, the United States’ increasingly permissive interpretation of the scope of the right of self defense under international law has begun to erode the international law prohibition on the use of force not only for the United States, but for the world at large. That, in turn, puts the global legal order, which is grounded in the prohibition of the use of force, at greater risk.

The Erosion of Congressional Constraints on War Powers

Over the course of the last century, presidential war powers have expanded to the point that, today, the president effectively exercises significant unilateral authority over decisions to use military force—at least in cases where there are no major deployment of ground troops. The U.S. Congress has not approved a use of force since 2002, when it authorized the president to invade Iraq. And yet the United States is still at war more than two decades later. In 2001, Congress authorized the United States to go to war against those who carried out the 9/11 attacks and any nation, organization, or persons that harbored them. Today, even as major combat operations have halted, the U.S. military is still in Afghanistan battling insurgent and terrorist forces. And the United States continues to fight insurgent groups that emerged in Iraq after the U.S. invasion. The U.S. government is also using force against extremist groups outside of Iraq and Afghanistan. The United States reportedly has active missions in around 20 countries, including most prominently in Afghanistan, Somalia, Yemen, Syria, and Libya.

None of these military operations has been separately authorized by Congress, even though the U.S. Constitution gives Congress the power to “declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water,” as well as to “raise and support Armies,” and “provide and maintain a Navy.” Instead, the legal basis for these operations is found in capacious readings of the congressional 2001 and 2002 Authorizations for Use of Military Force (AUMFs), together with an expansive interpretation of the president’s Article II authority to use force unilaterally. As a result, the vast majority of members of Congress have never voted to authorize a military operation.

One reason for the erosion of effective congressional constraints on the president’s power to unilaterally wage war was the fateful decision of the authors of the War Powers Resolution to tie the reporting requirements and the automatic withdrawal provisions in the Resolution not to “war” or “armed conflict,” but to “hostilities.” The House report on the War Power Resolution explained that the word was “substituted for the phrase armed conflict during the subcommittee drafting process because it was considered to be somewhat broader in scope . . . . [H]ostilities also encompass a state of confrontation in which no shots have been fired, but where there is a clear and present danger of armed conflict.” Perhaps because the meaning was self-evident to those involved, the term was not a subject of significant debate during the many hearings on the proposed legislation, nor was it defined in the legislation. That has since left it open to wildly differing interpretations. The problem has been exacerbated by the unwillingness of the courts to weigh in, relying instead on the standing and political question doctrines to avoid reaching the merits. That has often left the executive branch as the only source of public legal opinions on war powers—issued in the form of Office of Legal Counsel memoranda or testimony and speeches by Executive Branch lawyers.

Over the course of the decades following the passage of the resolution, administrations of both parties have adopted strained interpretations of “hostilities.” The War Powers Resolution was grievously ailing when the Obama administration dealt it what was arguably a death blow in 2011 by not seeking congressional authorization to continue military operations in Libya past the 60 day limit—and defending that decision as consistent with the War Powers Resolution because the intervention did not constitute “hostilities,” despite significant internal dissention within the Administration. The result of this narrow interpretation of hostilities by successive U.S. administrations has turned the constitutional order on its head—requiring Congress to muster supermajorities in both houses to stop U.S. involvement in a war rather than requiring the president to seek the support of Congress to start one. The result of years of decline in the role of Congress in decisions to use force abroad is that today the president is almost entirely unconstrained in waging war abroad.

The Rise of “Self Defense”

A major consequence of the erosion of Congress’ war powers is the expansion of the self-defense exception to the U.N. Charter’s prohibition on the use of military force. Due to the United States’ geopolitical role and its ability to articulate its legal positions broadly and forcefully, this shift has affected not only the law as applied to the United States, but also international law more broadly.

By its express terms, Article 51 of the United Nations Charter created a narrow exception for individual and collective self-defense where a U.N. member state has suffered an “armed attack.” The negotiating history supports this narrow reading. And yet there is little doubt that this understanding has come under intense pressure—much of it resulting from the U.S. executive branch’s increasingly aggressive interpretations of the exception. The United States is far from alone in offering stretched interpretations of Article 51. But as a key author and advocate of the Charter, a permanent member of the Security Council, and a state that holds itself out as committed to the rule of law with capacity to issue detailed legal justifications for its use of force decisions, the United States has an outsized impact on global understanding of the reach and limits of the self-defense exception. As its interpretation of the exception expands, other states follow. The rest of this section traces that evolution.

Preemptive Self-Defense—the “Bush Doctrine”

President George W. Bush Administration claimed the United States had the option to take preemptive action to counter a “sufficient threat to our national security.” In the case of significant threats, the United States could respond “even if uncertainty remains as to the time and place of the enemy’s attack.” Indeed, in the lead-up to the Iraq War, John Yoo wrote an opinion of the Department of Justice’s Office of Legal Counsel that claimed congressional authorization of the war was unnecessary, an assertion met with significant criticism at the time. The embrace of preemptive self-defense came to be known as the “Bush Doctrine.” President Bush’s successor, President Barack Obama, ran for office in part based on a more constrained view of presidential war powers. But once in office, Obama never renounced the expansive vision of self-defense that Bush had espoused. Indeed, he surprised his supporters and detractors alike by taking steps to expand it. Tellingly, while the Obama administration retracted John Yoo’s infamous legal opinions on torture, Yoo’s equally problematic legal opinions on the use of force still stand.

Self-Defense Against Non-State Actors

Before September 11, 2001, self-defense under Article 51 was generally believed to apply only, as scholar Monica Hakimi puts it, “in the case of an armed attack by one State against another State.” Examining the Harvard Law School’s compendium of Article 51 letters, it is clear that although a number of states had filed Article 51 letters in which they cited both state and non-state actor threats, relatively few states had exclusively cited non-state actor threats in Article 51 letters filed before 2001.

After September 11, 2001, non-state aggressors were more frequently treated as direct targets of self-defensive force by states. The United States in its Article 51 letter notifying the Security Council of its self-defense operations in Afghanistan after the 9/11 attacks indicated that it was responding to the “ongoing threat . . . posed by the Al-Qaeda organization” made possible by the Taliban’s willingness to allow Afghanistan to be used as a base of operation. Before the end of the year, the United Kingdom, Canada, France, Australia, Germany, New Zealand, and Poland had issued Article 51 letters that echoed the United States, relying explicitly or implicitly on an international law right of self-defense and collective self-defense against the non-state actors responsible for the September 11 attacks. A number of other Article 51 letters filed by states against other non-state actors followed in the next few years, and, indeed, Article 51 letters citing the right of self-defense against non-state actors have become common in the years since.

The United States soon endorsed what has come to be referred to as the “unwilling or unable” doctrine—the claim that action against a non-state threat is justified so long as the state in which the non-state actor resides is “unwilling or unable” to suppress the threat. The doctrine originated much earlier—during the Nixon administration, which claimed that self-defense was permissible whenever the neutral state “cannot or will not” prevent the unneutral use of its territory to justify expanding its military campaign against North Vietnamese forces in Cambodia. At last count, thirteen states either explicitly or implicitly endorse the “unwilling or unable doctrine,” six have objected to it, and ten have made ambiguous expressions. Latin American states, including Brazil and Mexico, have voiced opposition to the unwilling or unable doctrine, and the Community of Latin American and Caribbean States has expressed unease with the use of Article 51 to justify counterterrorism operations. The vast majority of states, however, have not expressed a position on the “unwilling or unable” doctrine—and it remains controversial outside the U.S. government, even if it is widely accepted within. Controversially, the United States appears to take silence on the issue as acquiescence, while others challenge that inference.

The assertion that Article 51 permits states to use armed force against non-state actors in a non-consenting state is in tension with the International Court of Justice’s (ICJ) decisions in Nicaragua v. United States, its Advisory Opinion on the Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, and Democratic Republic of the Congo v. Uganda. Nonetheless, for much of the last two decades, the United States and United Kingdom have publicly pressed the claim that military force against non-state aggressors like al-Qaeda is permissible under Article 51.

Former U.K. Legal Adviser Daniel Bethlehem outlined the U.S., U.K., and other allied governments’ legal position in a 2012 American Journal of International Law article published under Bethlehem’s own name, though widely understood to be based on conclusions reached in government-to-government talks known as the “West Point Group.” That article articulated a set of principles that have come to be referred to as the “Bethlehem Principles.” The first principle provides: “states have a right of self-defense against an imminent or actual armed attack by nonstate actors.” Part of what was notable about the principles, moreover, was the purpose of publishing them: Bethlehem acknowledged that they “do not reflect a settled view of any state,” but nonetheless indicated that “[t]he hope . . . is that the principles may attract a measure of agreement about the contours of the law relevant to the actual circumstances in which states are faced with an imminent or actual armed attack by nonstate actors.” In other words, the project was self-consciously aimed at development and evolution of the international law of self-defense.

The effort faced many critics. But even as the unwilling or unable doctrine remains contested, the United States has openly relied on it to justify expansive and longstanding uses of force that have perpetuated the so-called “forever war.” In doing so, it has set an example for others to follow.

“Collective Self-defense” on Behalf of Non-State Actor Partners

The United States’ latest step in the expansion of self-defense is the new assertion that the right of collective self-defense includes not only actions in defense of state actors, but actions in defense of non-state actors as well. The invocation of “collective self-defense” by the United States in this novel context could, over time, once again shift the scope of what is considered permissible under international law.

The novel theory that collective self-defense could be undertaken on behalf of non-state actor partners first became public in 2017, when the United States announced military operations in defense of the Syrian Democratic Forces (SDF), a coalition of ethnic militia and anti-government groups operating in North and East Syria with which the United States had partnered since 2015. On June 18, 2017, the United States shot down a manned Syrian regime SU-22 aircraft that had “dropped bombs near SDF fighters south of Tabqah”—a city in north-central Syria near Raqqa. According to a press release, this strike was in response to an attack by Syrian forces on Ja’Din, an SDF-controlled town, “wounding a number of SDF fighters and driving the SDF from the town.” It is unclear if any U.S. servicemembers were at risk, and the U.S. Department of Defense press release on the strike did not reference their presence. Instead, it stated, “At 6:43 p.m., a Syrian regime SU-22 dropped bombs near SDF fighters south of Tabqah and, in accordance with rules of engagement and in collective self-defense of Coalition partnered forces, was immediately shot down by a U.S. F/A-18E Super Hornet.”

Perhaps recognizing the extraordinary step it had taken—attacking the military forces of a country in its own territory in order to defend an unlawful organized armed non-state actor group—the press release went on to connect the attack to the ongoing self-defense mission in Syria: “[T]he Coalition’s mission is to defeat ISIS in Iraq and Syria. The Coalition does not seek to fight Syrian regime, Russian, or pro-regime forces partnered with them, but will not hesitate to defend Coalition or partner forces from any threat.” It further argued that “[t]he Coalition presence in Syria addresses the imminent threat ISIS in Syria poses globally. The demonstrated hostile intent and actions of pro-regime forces toward Coalition and partner forces in Syria conducting legitimate counter-ISIS operations will not be tolerated.”

The June 18 shoot-down was the first time a U.S. “warplane downed a manned aircraft since 1999,” during the NATO intervention in Kosovo, and it was the first time the United States justified force under international law as “collective self-defense of Coalition partnered forces” where that partner was a non-state actor and no U.S. servicemembers were apparently at risk.

In February 2018, the United States again cited the “defense of Coalition and partner forces” to justify “defensive strikes” against Syrian pro-regime forces. These justifications are notable because they indicate the U.S. government believed it could lawfully use force against Syria in collective self-defense of the SDF. In the years since, the United States has reflexively relied on “collective self-defense” in an array of circumstances that similarly push far beyond the historic boundaries of Article 51. For example, the United States has relied almost entirely on “collective self-defense” (this time of Somali counterparts) to justify strikes against al-Shabaab from 2015 through today. In characterizing the strikes as defensive in nature, U.S. forces were able to circumvent policy restrictions that would otherwise have applied. Indeed, the International Crisis Group quotes a U.S. official explaining that “[c]ollective self-defense is really close air support without authorization.” The rationale allowed military forces on the ground to get “ahead of what Washington had blessed or thought it had blessed,” and allowed more expansive strikes than would otherwise have been permitted under the 2001 AUMF.

These decisions to stretch the scope of Article 51 have been made, it is important to note, without any affirmative approval from Congress. Because the operations have been justified by the Executive Branch under the 2001 AUMF or in “self-defense,” they have not been submitted to Congress for approval. Nor have any formal legal memos articulating these positions been released to the public. Rather, one can only glean the evolving legal positions from speeches, press releases, and occasional public statements.

Other states have already begun to echo the United States. In 2019, Turkey—a NATO ally of the United States—launched an assault on Kurdish forces in northern Syria, an incursion it justified by “the right of self-defense as outlined in Article 51 of the U.N. Charter, to counter the imminent terrorist threat, to ensure Turkey’s border security, to neutralize terrorists.” And Russian President Vladimir Putin attempted to claim a right to act in the “collective self-defense” of the “independent” eastern regions of Ukraine when he initiated Russia’s war in Ukraine on February 24, 2022. The attempt at justification was met with widespread skepticism, as it should have been, but that Russia even considered the argument worth making reveals how flexible Article 51 has come to be seen.


The longstanding debate on reforming war powers and reasserting Congress’ authority over decisions to use force does not simply have stakes for U.S. constitutional law and separation of powers within the United States. The debate has global consequences. Reasserting congressional war powers is important to ensuring that the expansion of self-defense claims made by successive presidents does not continue. It is not simply a matter of constitutional law, it is a matter of international law as well. And, as such, it has implications not only for the scope of future U.S. actions, but for the behavior of other states who may rely on the same legal theories to take actions that undermine global peace and security.

Just as the erosion of Congress’ constitutional war powers have aided and abetted the erosion of constraints on Article 51’s exception for actions taken in self-defense, the opposite can also be true: Reaffirming Congress’s constitutional war powers could help reduce the likelihood that the United States will undertake military operations that are difficult, if not impossible, to square with the Charter. While taking steps to strengthen democratic checks on war is no guarantee that the United States will only use force consistent with international law in the future, reinvigorating Congress’s capacity to prevent the U.S. from going to war would make wars in violation of the Charter less likely. Moreover, a revised and revived War Powers Resolution that provides Congress real capacity to check presidential decisions to use force could serve a powerful symbolic function, demonstrating the United States’ commitment to the rule of law and serving as a counterpoint to the Russian state, which, it appears, has waged a disastrous illegal war of aggression on the whim of a single man. 

IMAGE: Military equipment at sunset (via Getty Images).