The United States has now been at war for more than two decades: nearly all this millennium, fourteen years longer than the Revolutionary War, and eighteen years longer than the Civil War or World War II. This “Forever War” began before a generation of today’s adults was born. Many of my current students are military veterans, a group that in the United States numbers in the millions. Countless Americans – to say nothing of those overseas who have borne the brunt of these interventions – have lived only in a state of war. But if war is meant to be aberrational, how did this happen (Part I today) and how do we end it (Part II tomorrow)?

I. Diagnosis

When I left the U.S. government as State Department Legal Adviser a decade ago (before returning again in 2021), I had thought hard about “How to End the Forever War.” A few months later, I summarized my conclusions at the Oxford Union and periodically updated my thoughts on this topic in the years since (here, here, and here). At Oxford, I argued that the United States should execute President Barack Obama’s plan to end the Forever War by leaving Afghanistan, closing Guantánamo, disciplining drones, and defeating al-Qaeda on the battlefield. A few weeks later, President Obama’s National Defense University speech embraced these same objectives and, at this writing, each seems to have been largely, if not entirely, achieved. The United States left Afghanistan, albeit sloppily, in August 2021. Of the 780 detainees held at Guantánamo at its peak, only 40 were left on January 21, 2023 – the day Joe Biden became President; today, only 30 detainees remain. To discipline drones and detention, first President Obama and then President Biden issued “Playbooks,” now largely declassified, to govern out-of-theater targeting, in the form of Presidential Policy Guidance (the “PPG”) and the stricter Presidential Policy Memorandum (the “PPM”), finally issued in October 2022. And the goal of defeating al-Qaeda on the battlefield mostly has been achieved, at least with respect to al-Qaeda Core.

But if these steps are largely completed, why does the Forever War appear to persist? Because societal, legal, and other social drivers have created pressures for continuing the war paradigm, as opposed to shifting to a law enforcement paradigm. In particular, sources of institutional tolerance – bureaucratic internalization of a war culture and the quiet normalization of “GWOT (Global War on Terror) Lite” – have combined with public apathy about whether the United States does, in fact, remain in a perpetual state of war. As a legal matter, overly broad interpretations of domestic and international laws, as well as government inertia against repealing outdated domestic laws, have contributed to making war the default paradigm.

The changing character of conflict has contributed to this state of affairs. Armed conflict is now waged by an array of state and non-state actors fighting on ill-defined battlefields and employing non-traditional uses of force, including through special operations, cyber tools, autonomous weapons systems, and artificial intelligence. Rapidly evolving technologies have helped conflicts to start, mutate, and persist at lower thresholds than full-scale war, making them more difficult to end.

Add in the evolving nature of the terrorist threat, which we understand much better now than we did on September 11, 2001. We now know that transborder terrorist groups splinter and spin off. They use social media and the “dark web” to recruit imitators around the world, who are usually not bona fide members of the original group but take pride in being associated with it. Their political aims are predominantly local, so they rarely launch attacks on major foreign capitals that are likely to trigger massive retaliation. And they do not surrender or concede defeat. Instead, as their membership depletes, less fervent members withdraw and cross borders as foreign fighters to join or form different groups with new names. Even when a terrorist group is effectively defeated on the battlefield, the core leadership of that group migrates and operates within a new armed group that constitutes a mobile threat from its new location.

The U.S. departure from Afghanistan and partial withdrawal from Iraq also raise the issue of “over-the-horizon” threats. Even when the United States or other countries withdraw from a military invasion – which may not have been wisely undertaken in the first place – they still must protect small military and civilian deployments left behind from local terrorist actors. These deployments must retain kinetic capacity for “unit self-defense” to defend against imminent attacks. Thus, continuous military deployments become an irreducible residue of past interventions, many of which may have been misguided from the start (such as the 2003 U.S.-led invasion of Iraq). In several countries – Iraq, Afghanistan, Syria, Yemen, and Somalia, to name a few – the constant fighting has created chaos and effectively destroyed meaningful governance in significant parts of those countries. Once chaos reigns in all or part of failed or failing states, zones of anarchy or “no-man’s lands” may become grounds for terrorist bases, particularly when the local sovereign is “unable or unwilling” to address the threat. And so a distant state that nevertheless perceives itself to be vulnerable to attack may feel obliged to extend its self-defense rationale to taking continuing military action to address these over-the-horizon threats, which helps perpetuate Forever War.

A swelling national security bureaucracy built to be laser-focused on these threats has fostered continuing financial and institutional incentives for entrenching the war paradigm. Within the executive branch, the national security bureaucracies have grown steadily richer, more powerful, and opaque relative to their diplomatic and justice counterparts. I personally witnessed this transformation over four stints in five decades in the U.S. government. On each return stint, I observed how – especially since 9/11 – the military and intelligence budgets have swelled, dwarfing that of the State Department, so that in the words of then Deputy Secretary of State William Burns, there are now “about as many members of the armed forces marching bands as there are American diplomats.” The 9/11 mentality has reshaped the foreign relations bureaucracy, with each agency replicating subunits that mirror and multiply a persistent focus on foreign counterterrorism.

The resulting bureaucratic structure too often resists new priorities in favor of combating more familiar threats. At interagency meetings, military and security priorities are regularly double-counted and kinetic solutions privileged over diplomatic ones. When I returned to the State Department as a Senior Advisor in 2021 at the start of the Biden administration – at a time when thousands of Americans were dying from COVID-19 and feeling the impacts of climate change, and just weeks after an angry domestic mob had attacked the U.S. Capitol seeking to undo a presidential election – countless hours were still being spent contemplating the continued detention of a few dozen aging detainees at Guantánamo and potential terrorist threats originating in distant theaters. This institutional fixation on past threats has resisted the Biden administration’s efforts to turn the page completely while having to address newly pressing challenges. So, when new urgent threats arise and demand attention – such as Russia’s 2022 invasion of Ukraine, China’s threat to Taiwan, climate change, and a global health crisis – bureaucratic inertia has tempered the Biden administration’s aspirations to fulfill old promises, such as ending the Forever War, closing Guantánamo, and repealing obsolete Authorizations for Use of Military Force (1991, 2001, and 2002 AUMFs). This continuing march toward executive unilateralism has been met by congressional acquiescence and judicial tolerance, ensuring that no branch will have the institutional incentives or wherewithal to finally end perpetual war.

All of these changes have blurred traditional lines in legal doctrine: between offense and lawful self-defense, both individual and collective; between uses of force and threats thereof; between proximate threats that trigger a lawful right of self-defense and remote threats that would not; between state and non-state actors; and between civilians and individuals who are lawfully targetable. This line-blurring also has dramatically affected governments’ perceptions of whether and when they have a legal right or duty to act to prevent serious harm to their own and foreign citizens.

The Forever War that began on 9/11 has also set legal and political precedents for other “frozen conflicts” around the world that appear to never truly end. Domestic political actors have begun to adapt so that war can now be waged remotely with manageable domestic consequences. When there are continuing hostilities but no surrenders, battlefield stalemate may lead to uneasy ceasefires, without ever ending in full-scale armistices, much less fully negotiated peace treaties – as, for example, in Kashmir, Nagorno-Karabakh; South Ossetia and Abkhazia; and North and South Korea. As Ukraine’s shaky counteroffensive proceeds, some have begun to ask whether the Russia-Ukraine War will join the list.

But if this is the grim diagnosis, on the 22nd anniversary of September 11, what should be the prescription? Part II, tomorrow, offers a concrete, long-term strategy for addressing the challenges that must be overcome to finally end this Forever War.

Editor’s Note: These two blogposts (Part I today and Part II tomorrow) derive from a keynote chapter in Brianna Rosen, ed., Perpetual War and International Law: Legacies of the War on Terror (Oxford University Press forthcoming 2024) and draw upon Harold Hongju Koh, The National Security Constitution in the 21st Century (Yale University Press forthcoming 2024), summarized in Harold Hongju Koh, “The 21st Century Nation Security Constitution,” 91 Geo. Wash. L. Rev. ___ (forthcoming 2023).

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