25 Years After 9/11: Lessons from the War on Terror and the Threats Ahead

Twenty-five years after the worst terrorist attacks in history, the United States is operating in a security environment that would have been difficult to imagine in 2001. The 9/11 attacks on the American homeland in 2001 established terrorism and transnational jihadism as central priorities and major threats to U.S. national security, and transformed how Washington understood intelligence, military power, and homeland security.

Emerging threats have now expanded beyond the transnational terrorist networks that defined the post-9/11 era. New military technologies, foreign interference, ransomware, autonomous systems, state-backed armed groups, and renewed competition with major powers are only a few of the threats that shape the current security environment. The Department of Justice, reflecting on the institutional changes that followed 9/11, similarly notes that the threat landscape has evolved to include challenges that were barely conceivable a quarter-century ago.

The past two decades offer no simple verdict on the War on Terror. However, they provide several lessons that remain directly relevant to the U.S. military as it prepares for a more complex and technologically contested era.

The Intelligence Lesson of 9/11

The attacks of September 11th did not occur in a strategic vacuum. Throughout the 1990s, al-Qaeda had demonstrated both its intent and growing capability to target U.S. interests, including the 1998 attacks against U.S. embassies in Kenya and Tanzania and the 2000 attack against the USS Cole.

The 9/11 Commission's findings contributed to major changes in intelligence coordination, information sharing, and institutional structures. The most consequential change that followed was institutional and operational. The United States developed an increasingly integrated model of intelligence-driven military operations. Special operations forces became central to counterterrorism campaigns, while persistent intelligence, surveillance, and reconnaissance enabled U.S. forces to monitor remote areas and identify individuals within dispersed networks. Precision strike capabilities allowed the United States to conduct operations against high-value targets without deploying large conventional formations in every case.

The result was an operational system that could combine human intelligence, signals intelligence, persistent surveillance, and special operations to target organizations such as al-Qaeda and later the Islamic State. That model produced important successes. It also created an institutional expectation that the United States should be able to detect and disrupt threats before they could generate strategic effects. Twenty-five years later, that expectation is being tested by a much more fragmented threat environment.

Afghanistan Demonstrated the Limits of Military Power as a Strategic Instrument

Afghanistan became the clearest example of the distinction between operational success and strategic outcomes. The initial U.S. objective after 9/11 was relatively focused: dismantle al-Qaeda's safe haven and remove the Taliban government that had provided the organization with sanctuary. Over time, however, the mission expanded into counterinsurgency, reconstruction, institution-building, and the development of Afghan security forces.

The U.S. military demonstrated an extraordinary ability to sustain operations in Afghanistan for two decades. Over the course of the war, nearly 800,000 U.S. service members have served in the country. The intervention, which cost 2,461 American lives, thousands of Afghan lives, and enormous financial resources, also marked the first and, to date, only invocation of NATO’s Article 5, transforming the response to the 9/11 attacks from the U.S.-led campaign into a collective alliance commitment. At its height, the NATO-led International Security Assistance Force (ISAF) included more than 130,000 personnel from 50 NATO and partner countries. During the 2009-2011 surge, American presence expanded significantly, reaching nearly 100,000 troops as the military sought to contain the insurgency and create the conditions for Afghan forces to assume greater responsibility.

The surge demonstrated that the U.S. military could adapt its tactics and apply significant force. Special operations forces and intelligence capabilities were used to target al-Qaeda and Taliban leaders, while persistent surveillance, airpower, and later increasingly sophisticated counterinsurgency operations allowed U.S. forces to disrupt insurgent networks and protect population centers. The mission also placed a growing emphasis on building Afghan military capabilities. NATO and U.S. forces trained and advised Afghan security forces, supported Afghan special operations forces and air capabilities, and worked to develop the logistics, intelligence, planning, and institutional capacity needed for Afghan forces to operate with greater independence. By January 2021, the U.S. military presence had fallen to 2,500 troops, with American forces focused primarily on counterterrorism and training, advising, and assisting Afghan security forces.

Yet, these operational gains did not translate into a lasting political settlement or a self-sustaining security system. The Special Inspector General for Afghanistan Reconstruction's lessons-learned work identified recurring problems in the reconstruction effort, including the absence of a coherent strategy, unrealistic timelines, and insufficient understanding of Afghan political and institutional conditions. Reflecting on NATO’s lessons from Afghanistan, John Manza –who served as NATO assistant secretary general for operations from 2018-2022– argues that

“[...]Had the allies kept their focus on the core goal of disrupting international terrorists in Afghanistan, the mission could have remained small and tailored to the unique conditions on the ground. A small NATO counterterrorism force, supported by a counterpart Afghan commando force, would have likely fit the bill.”

NATO’s own post-Afghanistan assessment reached a similar conclusion. It found that the international community’s level of ambition had extended far beyond the original goal of degrading terrorist safe havens and recommended that future operations continuously assess strategic interests, set achievable goals, and remain alert to the dangers of mission expansion. Afghanistan therefore exposed a problem that extends beyond counterinsurgency. The United States and its allies had demonstrated substantial capacity to conduct sustained military operations, adapt to changing battlefield conditions, and operate through a large coalition, but military effectiveness could not compensate for an unclear or ultimately unattainable political end state.

Future conflicts may again require prolonged campaigns, but the experience of Afghanistan demonstrates the importance of defining the political objective alongside the military mission, rather than allowing sustained military operations to become a substitute for a broader strategy.

The Next Threat May Not Look Like 9/11: Technological Diffusion and Disinformation

The 2022 war in Ukraine, Hamas’s October 7, 2023 attack on Israel, and Iran’s growing asymmetric capabilities are among the clearest examples of how military technologies and warfare have changed since 9/11. The United States now confronts terrorist organizations, state-backed armed groups, cyber actors, and increasingly sophisticated non-state networks that can exploit commercially available technologies. These actors may use drones, encrypted communications, social media, and artificial intelligence that either did not exist or were in their infancy when the War on Terror began. Taken together, these developments have fundamentally transformed the U.S. threat landscape, blurring the boundaries between conventional warfare, terrorism, cyber operations, and emerging technologies.

Drones provide one of the clearest examples of this technological diffusion. Unmanned systems that became central to U.S. surveillance and precision-strike operations during the War on Terror are now widely available to states, militias, and non-state organizations. The proliferation of relatively inexpensive drones has allowed weaker actors to conduct surveillance, disrupt military operations, and threaten critical infrastructure without requiring the resources traditionally associated with advanced military capabilities. In the United States, the scale of the challenge is already visible: the Federal Aviation Administration recorded more than 400 illegal drone flights over U.S. airports during the first quarter of 2025, while the U.S. Department of Defense has documented the growing threat posed by unmanned systems to military installations. In 2024, the Department of Defense described unmanned systems as an “urgent and enduring threat” to U.S. personnel, facilities, and assets, while emphasizing that drones are changing how wars are fought. Its strategy calls for counter-unmanned systems capabilities to become part of military doctrine, organization, training, equipment, personnel, facilities, and policy.

The FY2026 defense budget request includes $13.4 billion for autonomy and autonomous systems, including $9.4 billion for unmanned and remotely operated aerial vehicles and $15.1 billion for cyberspace activities -which is 4.1% more than the previous year's request- to counter unacceptable cyber threats eroding US and Allies’ military advantage. The Department of the Navy requested $5.3 billion for unmanned systems and $1.3 billion for counter-UAS capabilities, while the Army requested more than $1.1 billion for unmanned capabilities such as short-range reconnaissance, long-range reconnaissance, and launched effects. The Army is also combining funding for counter-small UAS, unmanned systems, and electronic warfare in an effort to respond more quickly to technologies that are evolving rapidly on the battlefield.

For service members, this means that drones and counter-drone systems are increasingly becoming part of ordinary military operations and training. The military's Joint Counter-Small UAS University now trains Soldiers, Sailors, Airmen, and Marines to operate counter-drone equipment, plan defenses, and prepare their units for UAS threats. At the Army's National Training Center, commercial and military drones are incorporated into training scenarios involving surveillance, targeting, and simulated attacks. Joint Interagency Task Force 401 has expanded counter-UAS training as part of its homeland defense mission, reflecting the growing need for military personnel to recognize, track, and respond to drone threats. The task force's work also brings together military and interagency capabilities, illustrating how countering unmanned systems is becoming a broader force-protection requirement rather than a problem confined to traditional air-defense units.

Communications, GPS, electronic warfare, and cyber capabilities are becoming increasingly important to everyday operations as adversaries look for ways to disrupt the networks and systems on which U.S. forces depend. As the security environment has become more interconnected, the military has also had to rethink how it protects the systems that support its operations. The Department of Defense’s Zero Trust strategy calls for a shift toward a cybersecurity architecture designed to continuously verify users, devices, and network activity rather than relying primarily on traditional network boundaries. At the same time, the department has placed greater emphasis on protecting U.S. critical infrastructure and defense industrial base partners from malicious cyber activity. Navy experimentation is bringing unmanned systems together with cyber, electronic warfare, communications and networking technologies rather than treating them as separate problems.

This technological diffusion has also transformed the information environment in which military operations take place. Artificial intelligence may accelerate this trend by making propaganda, disinformation and other forms of information manipulation faster, cheaper and more difficult to detect. The Council on Foreign Relations (CFR) assessment warns that terrorist groups are increasingly positioned to exploit AI for propaganda, disinformation, recruitment, surveillance and other operational purposes, even as governments seek to use the same technologies to improve threat detection and prevention. Disinformation has also become an important instrument of state power for US adversaries. According to Kovalčíková Nad’a, a senior analyst at the European Union Institute for Security Studies,

“The Kremlin annually spends about USD 1.5 billion on propaganda beyond its borders, a highly asymmetric volume compared with the EU and its allies’ defensive measures in the metastasizing ecosystem of foreign interference threats.”

In the Middle East, Russian Arabic-language media have sought to exploit existing distrust of the United States and NATO by promoting narratives that portray Moscow as a counterweight to the Western-led international order while emphasizing Western interventionism and perceived inconsistencies in U.S. foreign policy. These campaigns do not necessarily need to persuade audiences to support Russia; by reinforcing existing grievances and distrust, they can weaken confidence in the United States and its partners. For the U.S. military, this means that the operational environment is no longer limited to the physical battlefield. The ability to identify, understand, and counter information operations is becoming increasingly important to military effectiveness, particularly as adversaries combine technological capabilities with efforts to shape how conflicts are perceived by local populations, allies, and the American public.

In testimony before the House Permanent Select Committee on Intelligence in July 2026, Seth G. Jones, president of the Defense and Security Department at the Center for Strategic and International Studies, stated that,

“The international landscape, the threats to the United States, and intelligence challenges have dramatically evolved” since the 9/11 attacks.

Jones argues that the United States and its allies now face an “authoritarian axis led by China that also includes countries such as Russia, Iran, and North Korea,” alongside terrorist groups and other state and non-state actors.

He also identified the growing use of

“...offensive cyber operations, autonomous and AI-enabled unmanned systems, espionage, and AI-enabled influence campaigns on social media” as part of the evolving threat to the U.S. homeland.

Twenty-Five Years Later

In the aftermath of the 9/11 attacks, U.S. foreign policy was defined by a clear priority: prevent another catastrophic terrorist attack and dismantle the organizations capable of conducting one. Twenty-five years later, terrorism remains part of the threat landscape, but it operates alongside cyber threats, autonomous systems, state-backed armed groups, and technologies that have lowered the barrier to conducting sophisticated operations.

The experience of the recent war with Iran also highlighted another lesson: military advantage cannot depend exclusively on increasingly expensive and technologically complex systems. The effectiveness of low-cost drones and other relatively inexpensive capabilities against much more costly weapons has reinforced the need to rethink how the United States balances exquisite systems with affordable, scalable ones. As Doug Beck argues in Foreign Affairs, the military revolution is not simply about developing new technology but about whether U.S. institutions can adapt their acquisition, production, and operational models quickly enough to keep pace with it. The 2025 National Security Strategy similarly argues that preventing future conflict will require “a renewed defense industrial base” and winning “the economic and technological competition over the long term.”

The lesson of the past 25 years is that adaptation must become a permanent strategic requirement. The capabilities developed after 9/11 remain valuable, but the next phase of U.S. military power will depend on combining technological superiority with affordability, speed, and the ability to produce and deploy capabilities at scale.