The Lessons Learned for U.S. National Security Policy in the 20 Years Since 9/11

Americans will never forget the coordinated terrorist attacks on September 11, 2001, against the country’s financial center in New York, its global military headquarters at the Pentagon, and its civilian air transportation system. The images of New York police and firefighters rushing into the burning World Trade Center to rescue those trapped—and the memory of their sacrifices—will not fade from memory. America, its NATO allies, and the U.N. Security Council responded with swiftness and ingenuity to protect its citizens in the immediate aftermath and deployed measures to protect the their citizens, established new government agencies, and undertook numerous military operations overseas intended to eliminate threats and enhance stability.

This week’s 20-year marker of 9/11 offers an opportunity to reflect and take account of how the world and the United States has changed over the course of this generational time span. However, any assessment aiming to weigh the costs and benefits of U.S. national security responses and approaches across four U.S. administrations will offer only a partial snapshot.

The protective actions taken over 20 years have produced important gains in security at home—but these gains came with major human, financial, and strategic opportunity costs. The United States led international coalitions into three major wars in Afghanistan, Iraq, and against the Islamic State group in Syria and Iraq; it also conducted military and intelligence operations in dozens of other countries. Admittedly, tallying gains against costs yields inconclusive results. What emerges clearly, however, is the consistency with which some of the biggest challenges emanate from unforced U.S. policy errors and unpredictable consequences of well-intended actions.

Today, the United States is more secure on the home front from foreign terrorist attacks yet faces increased domestic terrorist threats. America’s efforts to promote freedom and democratic governance in the world faltered. Freedom globally has stagnated and deteriorated since 2005, and the significant stresses on America’s own democratic system have risen dramatically in recent years.

The strategic ledger accounting for 20 years of effort remains decidedly mixed. There have been undeniable gains and signs of progress, but they came at great costs and too often spawned new challenges. However, there are lessons to be learned as the United States looks toward a national security strategy for the next two decades.

Gains and signs of progress

The United States is safer from foreign terrorist network threats

A total of 2,977 people were killed in the 9/11 attacks, the largest single death toll from a foreign attack on America. Since then, 107 Americans have died in Salafi-jihadi terrorist incidents in the United States.

In the wake of 9/11, no additional major foreign terrorist attacks took place within the United States itself. This is the result of multiple lines of effort. The United States created new institutions, including the Department of Homeland Security, Directorate of National Intelligence, and National Counterterrorism Center. It substantially increased resources for intelligence and law enforcement agencies as well.

In addition, the United States implemented new security procedures to restore confidence in air travel. It developed new military and intelligence capabilities that helped thwart numerous plots and track down terrorist operatives and leaders, most notably Osama Bin Laden in 2011. The U.S. substantially enhanced its capacities to track and shut down terror finance networks, and it adapted its approaches to address terror threats in the cyber realm, staying in front of rapidly changing technologies. U.S. military and intelligence agencies created or strengthened counterterrorism partnerships across the globe, and these joint efforts helped the United States achieve more than it would have on its own.

These military, intelligence, and law enforcement efforts produced a key result: enhancing security at home. But some of these initiatives came with substantial financial, moral, and strategic opportunity costs.

America recognized the need to redefine national security and elevate nonmilitary tools of national power but fell short in making a fundamental shift toward a new approach

A few years into the initial U.S. policy response to the 9/11 attacks, the United States started to recognize the limits of a military-centric approach, with many political leaders, policymakers, and analysts calling for prioritizing diplomacy, economic tools, and political and ideological engagement. This growing recognition led to modest reforms, but the main structures and resources dedicated to so called “hard” security remained front and center. Efforts to integrate “smart power” as a central concept in U.S. national security failed to achieve the promised and desired results. Too often, the United States repeated past patterns of behavior in not using foreign assistance and other related tools effectively to achieve U.S. policy objectives.

A new generation came of age in more open societies in Iraq and Afghanistan

The wars in Afghanistan and Iraq removed authoritarian rulers from power and produced a qualified and tenuous sense of freedom in those countries that remained vulnerable to many security threats and challenges. The costs of these wars to these societies, America, and other coalition partners were substantial. But one gain that will endure are new generations of Afghans and Iraqis born after the start of these wars and who grew up in imperfect but more open societies. This new generation has shown a desire for positive change, as witnessed by the numerous waves of protest movements in Iraq in recent years and the continued participation in an open but imperfect electoral system. And while recent events in Afghanistan cast a dark shadow on these generational gains, Afghan youth—and women who had access to education and leadership roles—will likely make it more difficult for the Taliban to reimpose an old order.

Costs and sacrifices

The human costs and casualties of wars and conflicts have been staggering

It is difficult to capture the costs to human life in a wide range of efforts over two decades in dozens of countries. In post-9/11 military operations, the United States lost 7,074 troops killed in action and another 53,303 wounded. A further 1.8 million post-9/11 veterans have reported service-connected disabilities to the Department of Veterans Affairs. Furthermore, American allies and coalition partners lost 1,519 troops killed in action in Iraq, Afghanistan, and military counterterrorism operations elsewhere.

In addition, 18 American diplomats and aid workers were killed Middle East and South Asia since September 11, including seven in Iraq and three in Afghanistan. Twenty United Nations diplomats and workers were killed in attacks on UN facilities in Iraq and Afghanistan in 2003 and 2009. At least 10 diplomats from allied or coalition partner nations including Japan, Romania, the United Arab Emirates, Turkey, and Egypt were also killed in Iraq and Afghanistan over the past two decades.

Most estimates place the total number of deaths due to conflicts in Iraq, Afghanistan, and other countries part of the post-9/11 wars in the hundreds of thousands, including both civilians and combatants. However, it remains difficult to calculate precise numbers of those killed and wounded due to gaps and uncertainties in reporting.

Direct financial costs for U.S. taxpayers in the trillions

The total direct costs of the post-9/11 wars in terms of U.S. government spending are roughly $2 trillion over 20 years, including direct war costs and money spent on reconstruction, humanitarian aid, and efforts to build and support local security forces. This figure represents about 2.7 percent of the total U.S. federal government spending of more than $74 trillion during this 20-year period, a period when the total U.S. GDP was more than $335 trillion. These direct war and reconstruction costs amounted to less than 1 percent of overall GDP.

Large strategic opportunity costs

The 20 years of military operations and other efforts responding to the 9/11 attacks and other terrorist threats including the Islamic State group took time, resources, and attention away from other efforts needed to protect and strengthen the United States. In the international system, new transnational challenges such as pandemics and cybersecurity emerged, and state competitors such as China and Russia adopted more assertive stances in the global arena. As America focused on the battle against terrorist networks, the broader global landscape shifted.

While the base defense budget remains roughly similar as a share of GDP as it was before 9/11—2.78 percent of GDP in 2001 versus 2.86 percent of GDP in the Biden administration’s most recent budget request—the Department of Defense consistently invested in immediate priorities related to the wars in Iraq and Afghanistan rather than capabilities to counter longer-term challenges such as Russia or China. Secretary of Defense Robert Gates, for instance, stopped production of the F-22 Raptor stealth fighter in part because he saw it as useless in the wars in Iraq and Afghanistan.

A faltering struggle for freedom abroad begins to impact America at home

In the wake of the 9/11 attacks, many voices across America’s political and ideological spectrum backed a call to support freedom and democracy to counter the extremist ideas that fuel terrorist networks. The Bush administration initially framed its “Global War on Terror” in terms of a “Freedom Agenda,” and it justified some of its moves, including the 2003 Iraq War, within that framework.

Yet missteps and unforced errors undermined America’s efforts to provide itself as an example of moral leadership in the world. This included high profile abuses, including the torture of detainees at prisons in war zones and around the world; controversial drone strikes; and broad intelligence collection programs that used new technologies to collect information in ways that raised questions about the checks and balances of America’s democratic political system.

Moreover, after decades of gains for freedom in the world beginning from the late 1970s, the tide started to turn in and around 2005. During the ensuing years—roughly over the past decade and a half—the world has witnessed stagnation and declines in political rights and civil liberties worldwide. In the Middle East and parts of South Asia, for example, despite enormous churn and shifts in power—including a series of protests starting in the 2011 Arab uprising and continuing until today—the fundamental picture for basic rights and freedoms remains bleak, with nearly all countries ranked as “Not Free” or “Partly Free” by Freedom House.

During the past five years inside of the United States, internal challenges to the country’s democratic system and mores emerged, with growing concerns about the stability and legitimacy of the election system and the broader system of government.

The battle of ideas is still not won and extremist ideologies and terrorist networks continue to take on new forms and spread

The extremist ideologies that were present 20 years ago have evolved, taken on different forms, and spread across the Middle East, Africa, and Asia.

Globally, the fight against Salafi-jihadi terrorism presents a mixed picture. U.S. special operations forces killed Al-Qaida leader Osama bin Laden in 2011. The United States has also had success in killing major terrorist leaders beyond bin Laden, including al-Qaida in Iraq’s Abu Musab al-Zarqawi; al-Qaida in the Arabian Peninsula’s Anwar al-Awlaki; and the Islamic State group’s Abu Bakr al-Baghdadi. America’s drone campaign against al-Qaida on the Afghanistan-Pakistan border appears to have proven successful in significantly degrading the core group’s capacity to carry out attacks. Likewise, the Islamic State group was deprived of its territorial control by a U.S.-led military campaign in Iraq and Syria.

Though groups such as the Islamic State group commanded the world’s attention in recent years, its Salafi-jihadi ideology has metastasized worldwide, with groups emerging in locations as seemingly unlikely as Mozambique. Terrorist groups motivated by this ideology have killed thousands in sub-Saharan Africa and the Asia-Pacific region over the past two decades, including the 2002 Bali bombing and 2008 Mumbai hotel siege that left hundreds dead.

The fall of Afghanistan to the Taliban following the U.S. military withdrawal likely represents a significant setback. The Taliban will likely provide safe harbor to al-Qaida and other Salafi-jihadi terrorist groups moving forward, and the U.S. military and intelligence community will face greater difficulty thwarting terrorist attacks with a hostile government in power in Kabul. As with the rise of the Islamic State group, it could give Salafi-jihadism an infusion of new energy.

Failed states continue to strain the international system

Chronic instability and a lack of effective governance in key parts of the Middle East, South Asia, and Africa leaves countries vulnerable to nonstate threats. This is not a necessary consequence of the past 20 years of U.S. action. Rather, it was a preexisting condition that did not improve.

Initiatives aimed at helping other countries produce legitimate and effective governing authorities that were a part of the post-9/11 efforts in Afghanistan, Iraq, and other key countries failed to produce a major example of success. The weakest link in the post-9/11 efforts centered around the challenge of helping other societies develop governments that were effective and had broad legitimacy in the eyes of their own people. This complicated task involved a deep understanding of internal power dynamics and determining the best way to help other societies navigate major divisions—an understanding that the United States and other outside actors never fully acquired.

A growing global refugee crisis presents additional challenges for the world

Because of the persistence of extremist ideologies, violent conflict, and state failure, millions of people are on the move around the world. Overall, the global refugee and internally displaced persons (IDPs) population has gone from a combined 18 million (12 million refugees and 6 million IDPs) in 2000 to a combined more than 69. 2 million (20.6 million refugees and 48.5 million IDPs) in 2020—an almost 350 percent increase.

A multiplicity of factors has contributed to this growing crisis, including climate change and conflicts not directly linked to the challenges of terrorist networks and the post-9/11 wars. But, as witnessed in recent weeks in Afghanistan and over the past few years in conflicts in Syria, Libya, and Yemen, state failure and the presence of nonstate terrorist groups often drives people to flee across borders.

The inconclusive results of 20 years of war after 9/11 contributed to a deeply divided and polarized nation

America’s political leaders consistently failed to explain how the country’s post-9/11 wars fit into their overall foreign policy visions. While President George W. Bush inconsistently attempted to frame the Iraq war as part of his Freedom Agenda in the Middle East, his three successors dwelled on the failures of the U.S. post-9/11 wars and its general engagement with Middle East. At the same time, political actors sought to turn these wars into partisan wedge issues—starting with President Bush’s push to invade Iraq during the 2002 mid-term election cycle and continuing through to the Benghazi consulate attack in 2012 and the fight against the Islamic State group from 2014 to 2017.

10 lessons learned for U.S. national security policy from 2001 to 2021

  1. America remains an exceptionally wealthy nation with a resilient economy that can financially afford to meet threats overseas and pursue ambitious reform and public investment programs at home. The wars in Iraq and Afghanistan cost less as a share of America’s national economy than the wars of the previous century. Spending for World War II peaked at more than a third of the national economy in 1945, while wars in Korea and Vietnam peaked at 4.2 percent and 2.3 percent of the national economy in 1952 and 1968 respectively. By comparison, spending on both the wars in Iraq and Afghanistan peaked at 1.2 percent of the national economy in 2008. At the same time, the United States enacted expensive and ambitious policy programs at home: the 2001 and 2003 Bush tax cuts; Medicare drug coverage expansion in 2003; the 2008 financial bailout; the 2009 Obama stimulus; the 2010 Affordable Care Act; the 2017 Trump tax cuts; and a series of multitrillion dollar COVID-19 relief packages in 2020 and 2021. These polices were all pursued regardless of the financial cost of wars in Iraq and Afghanistan, and rarely were the costs of these wars used to characterize these domestic policies as unaffordable.
  1. The United States too often overestimates benefits and underestimates costs of particular policy choices. There is an important similarity between how the United States got into Iraq in 2003 and how it left Afghanistan in 2021: Political leaders and policymakers overestimated the benefits and underestimated the costs of preferred courses of action. In making the case for these policies, the presidents involved offered a set of false choices: invade or be attacked with weapons of mass destruction; withdraw from a conflict or escalate it. Arguably, a broader range of strategic options were available to meet the stated challenges than put forward by America’s political leaders.
  1. The U.S. government has much room for improvement in synchronizing and integrating its tremendous diplomatic, military, security, and economic resources to achieve maximum effect in support of America’s foreign policy. Despite plenty of talk about “smart power” and “integrated power” over decades, the United States has yet to discover an effective way to synchronize and integrate all elements of national power. “Civilian surges” in Iraq and Afghanistan failed to produce promised results, and policymakers increasingly relied on the military to carry out tasks better handled by civilian agencies such as the U.S. Agency for International Development. Even as political leaders and policymakers publicly committed to use diplomacy and other nonmilitary tools as a first resort, they lacked the ability to use those tools effectively or in concert. They also remain understandably risk-averse when it comes to civilian agencies operating without the U.S. military protection in unstable environments—a dynamic reinforced by the domestic political response to the Benghazi consulate attack in 2012. However, such posture dramatically reduces the options for credible commitments or implementation of American economic resources. Furthermore, the United States should set more realistic and achievable outcomes and set expectations at home and overseas. It should not promise more than it can deliver, and it should make sure it delivers on what it promises. Maximalist policies—whether implicit or explicit—almost always fall short of expectations. There is no “one-size-fits-all” approach to reconstruction and stabilization. More patient and less ambitious reconstruction efforts could prove more successful and sustainable—and less expensive. The desire for speed, especially to expedite the departure of U.S. military forces via an “exit strategy,” makes reconstruction efforts more expensive and less successful. Moreover, counterinsurgency doctrine in particular seduced many policymakers after its perceived success in Iraq, leading to its application in Afghanistan and a failure to recognize its shortcomings in Iraq.
  1. Corruption within partner governments and U.S. operations undermines effectiveness and credibility. Waste, fraud, and abuse are persistent challenges in wide-ranging, extensive U.S. operations. When U.S. objectives include helping other societies construct effective and legitimate governments, America should be extra vigilant in ensuring necessary accountability and transparency. The United States squandered billions of dollars in both Iraq and Afghanistan due to lack of effective management, oversight, and controls, and consistent internal and external pressure to deliver substantial resources. As the Special Inspector General for Afghanistan Reconstruction has noted, large injections of reconstruction spending over short periods of time can fuel local corruption and prevent the quick implementation of a U.S. exit strategy.
  1. The United States needs to better attune its military and economic assistance to local realities. In Afghanistan, for instance, the United States built a military in its own image—dependent on airpower and contractors that also departed when the U.S. military withdrew. This has not worked for the United States elsewhere, and it did not work in Afghanistan or Iraq. Reconstruction efforts were too often geared toward expediting a U.S. departure rather than toward local necessities, frequently fostering corruption in the process and yielding inefficient returns on investment. Security and economic assistance efforts should be attuned to local conditions and needs to achieve better outcomes.
  1. Ending American military involvement in conflicts requires better planning and forethought. Military withdrawal generally does not end American involvement in conflicts. President Obama withdrew from Iraq in 2011 only to reengage when the Islamic State group emerged in 2014. Likewise, President Joe Biden pulled U.S. troops from Afghanistan but has pledged to maintain a still-undefined “over-the-horizon” counterterrorism mission focused on the country. Promises of post-withdrawal U.S. civilian engagement have also failed. The Obama administration formulated ambitious plans for post-withdrawal American civilian engagement in Iraq, but these plans were never realized. Likewise, in Afghanistan, the Biden administration’s pledges of continued post-withdrawal engagement were never put to the test—and its lack of urgency surrounding the safe evacuation of Afghans who assisted the U.S. war effort suggests it would have followed the same path as the Obama administration in Iraq.
  1. Allies and partners are effective force multipliers if and when the United States works to improve coordination and relationships between them. Oftentimes over the past two decades, the United States seems to have taken its allies for granted despite their contributions and sacrifices in Iraq, Afghanistan, and elsewhere. This attitude has left the United States unable to coordinate and leverage its relationships with its allies and partners around the world as well as it could. Better coordination with clearly delineated roles for the United States and its allies could help achieve better outcomes and will be necessary going forward.
  1. The ideological battle remains an important component in the challenge to counter extremism and terrorism. Critics have deemed U.S. political and “nation-building” efforts in Iraq and Afghanistan failures because they failed to transform these countries into full-fledged democracies within a short period. While societal transformation at such speed and scope was an unrealistic objective, these efforts have allowed a generation in Iraq and Afghanistan to reach adulthood in far more liberal political circumstances than their parents and grandparents. Those of this post-2001 generation remain under severe threat, especially in Afghanistan. Whether the opening of near-term opportunities for so many individuals will last over the long term remains uncertain. But the persistence of political activism among many young Iraqis despite assassination campaigns by Iranian-backed militias, for instance, makes clear that American political and nation-building efforts did in fact have some positive, if unpredictable, effects in Iraqi and Afghan societies.
  1. Customs and immigration policies adopted after 9/11 were mixed at best and deserve careful, regular review. In the years that followed 9/11, discussions of immigration policy shifted significantly away from comprehensive reform as the United States tightened restrictions and imposed new requirements on foreigners wishing to enter the country. Some programs, such as the US-VISIT biometrics collection initiative, became part of a broader biometric entry-exist program, while others, like the National Security Entry-Exit Registration System (NSEERS), were curtailed and then discontinued after criticism of their effectiveness and illiberalism. Other international customs inspection programs such as the Container Security Initiative have also been put in place at ports of entry around the world, from Dubai to Shanghai. These programs require regular review to ensure that they are in fact effective and do not compromise fundamental American values. To meet today’s challenges, moreover, theprioritiesof the Department of Homeland Security—the lead immigration enforcement agency—should be rebalanced to improve the treatment of those arriving in the United States.
  1. America’s geopolitical competition with countries such as China and Russia, and global perceptions about U.S. power, influence, and credibility are affected by outcomes in places such as Afghanistan and Iraq. During the four U.S. administrations since 9/11, the United States has too often fallen short in achieving its stated outcomes in the fight against terrorist networks and the broader efforts to achieve stability in places such as Iraq and Afghanistan. The crisis of effectiveness raises doubts about America’s ability to accomplish what it sets out to do. These perceptions about America’s effectiveness spillover to other regions and countries. They must be factored into U.S. foreign policy decision-making moving forward.

America’s post-9/11 foreign policy yielded some important successes, most notably the prevention of a major foreign terrorist attack on the United States. But these more positive outcomes came at great strategic, material, and human cost—including the creation of a number of conflicts and foreign policy crises that will not disappear in the near future.

Brian Katulis is a senior fellow at the Center for American Progress. Peter Juul is a senior policy analyst at the Center for American Progress.