The Lessons of 9/11 for Defense Planning

September 8, 2021  |  Dr. Roger Cliff


On the morning of September 11, 2001, I was working at the Pentagon in the office of the deputy assistant secretary of defense for strategy. The DoD was preparing to issue a report documenting the findings of the 2001 Quadrennial Defense Review, an eight-month-long, comprehensive review of the department’s plans and programs. The theme of the report was the need to “transform” the armed forces of the United States to ensure they would continue to hold a dominant advantage over any potential, future challenger. Although never named in the report, the challenger DoD leaders had in mind was China.

The United States at the time was an unrivaled power in the world. The collapse of the communist regimes in Eastern Europe in 1989 and the breakup of the Soviet Union in 1991 had left the United States the world’s sole superpower. The armed forces of Russia, the Soviet Union’s successor state, were decrepit and ineffectual, unable even to defeat rebels in Russia’s breakaway republic of Chechnya in a 1994–96 war. Meanwhile, the 1991 Persian Gulf War had demonstrated the ability of the United States to easily eviscerate the conventional forces of any regional power that might think to threaten US interests. At the time, international relations scholars referred to this period as “the unipolar moment.” One of my coworkers in the strategy office drew a graph illustrating the growth of America’s relative power since World War II. Essentially, the figure depicted an exponential growth curve, exploding toward infinity over time.

The most challenging, potential, near-term scenario defense planners could imagine in 2001 was the possibility of the simultaneous occurrence of two regional wars comparable to the 1991 Persian Gulf War, and this possible contingency formed the basis for assessing the adequacy of the size and capabilities of US forces. The principal concern of US military leaders at the time, however, was not the possibility of two major, regional conflicts; rather, the principal concern was the stress on US armed forces caused by the need to support ongoing peacekeeping deployments in former Yugoslavia and elsewhere around the globe—or, in the case of the US Navy, the need to meet self-identified “presence” requirements in different regions of the world. Although China was recognized as having the economic and technological potential to become a rival on a scale comparable to the Soviet Union during the Cold War, in 2001, this prospect was largely hypothetical. Indeed, many observers viewed the DoD’s focus on the possibility of China being a future adversary as the result of the military-industrial complex’s need to justify continued, massive defense budgets.

The George W. Bush administration entered office promising to revitalize the US armed forces after the supposed neglect of the previous administration. In a world where US dominance was unchallenged, however, increased defense spending was not a priority for either the administration or Congress. In the summer of 2001, when Secretary of Defense Donald Rumsfeld approached the White House about requesting an increase in defense spending to carry out his vision of transforming the US military, he was rebuffed and told to expect no significant increase.

All of these beliefs and concerns changed on the morning of September 11, 2001, when two hijacked airliners were crashed into the World Trade Center towers, a third airliner was crashed into the Pentagon, and a fourth went down in Pennsylvania after passengers fought with the hijackers over control of the airplane. International terrorism had previously been relegated to lists of other security concerns, along with issues such as the proliferation of nuclear, biological, and chemical weapons; drug trafficking; and organized crime. Similarly, in an effort to counter what was seen as the excessive dispersal of US defense resources and attention across the world, during the Quadrennial Defense Review, DoD leaders specified the United States would concentrate its forces and defense planning on four regions of the world that were of vital importance to the United States: Europe; Northeast Asia; the Middle East and Southwest Asia; and “the East Asian littoral,” which stretches “from south of Japan through Australia and into the Bay of Bengal.”1 Before the report from the review had even been published, however, terrorism had become the DoD’s top security concern and the United States was at war in a place outside of these four regions.

The United States has been engaged in combat operations more or less continuously in the 20 years since the September 11 attacks. More than 5,000 Americans have been killed in military action; more than 50,000 have been wounded in action, many of whom have been maimed for life; and untold numbers who have suffered psychological injuries.2 Foreign deaths, mostly civilian, in the various operations associated with the war on terrorism have been estimated at more than half a million, not including enemy combatants.3 And yet, 20 years after having been ousted by the United States, the Taliban has retaken control of Afghanistan, and a 2017 CNA assessment of al-Qaeda, the organization behind the September 11 attacks, concluded, “Al-Qaeda today is larger, more agile, and more resilient than it was in 2001.”4 Meanwhile, China’s economy, which was less than half the size of that of the United States in 2001, is now 16 percent larger, and China’s annual defense spending, which was less than one-eighth the size of the US defense budget in 2001, is now the second highest in the world and roughly equal to that of all other Indo-Pacific countries combined.5

But describing the decisions made by US leaders before and after the 9/11 attacks as mistakes or the sacrifices made by Americans and their allies to have been in vain would be inaccurate. Nearly 3,000 people died in the terrorist attacks of 9/11, and the number could easily have been much higher if a concomitance of circumstances had not enabled a significant proportion of the occupants of the World Trade Center towers to evacuate before they collapsed. Meanwhile, the actions taken by the United States since 9/11 have apparently been successful in preventing an attack of comparable magnitude from occurring. Indeed, despite predictions in the immediate aftermath of 9/11, the incidence of terrorism in the United States has been significantly lower than it was in the years prior. From September 12, 2001, to December 31, 2019 (the last day for which comprehensive data is available), for example, an average of about 30 Americans died in terrorist attacks each year.6 By comparison, between January 1, 1994 (the first day for which comprehensive data is available), and September 10, 2001, an average of about 67 Americans were killed in terrorist attacks each year.7 Many of the American terrorist deaths that have occurred since 9/11, moreover, have not been caused by Islamist organizations like al-Qaeda. Thus, though al-Qaeda may still survive (although Osama bin Laden, its leader on 9/11, does not) and the Taliban may have returned to power, Americans have largely been safe from Islamist terrorist attacks since 9/11.

Not all of the actions taken by the United States since 9/11 in the name of making Americans safer, such as the invasion of Iraq, have actually contributed to this goal, and the sudden collapse of the Afghan government and armed forces the United States spent 20 years building is devastating, not least for the people of Afghanistan. In aggregate, however, the efforts made since 9/11 to protect Americans from terrorist attacks appear to have been largely successful. Soldiers and civilians in the United States who have sacrificed and risked so much in conflicts around the world and in efforts to reconstruct Afghanistan and Iraq should take pride in having been part of a campaign that, though flawed and needlessly costly, nonetheless achieved its primary objective.

At the same time, the US leaders who in 2001 recognized China’s potential to emerge as a major threat to the free world and the need to maintain America’s military dominance were not wrong. China today is far more powerful than it was in 2001, more repressive internally, and more hostile toward the world externally. The Afghanistan War and the Iraq War, which were precipitated by the 9/11 attacks, resulted in both enormous human costs and a major diversion of resources away from the modernization needed to maintain the US military advantage over China. Some modernization efforts continued, however, and they provide the foundation for a renewed focus on countering China’s military growth.

The events of September 11, 2001, and the 20 years that have followed show national security threats can appear without warning from unexpected directions, and the US armed forces may be required to perform missions that they have no intention or desire to perform. In other instances, however, threats may come from an obvious source that signals its intentions long in advance. Prudent planning, therefore, requires the US military to maintain some capability and expertise to respond to all plausible contingencies, not just those on which it would prefer to focus. At the same time, when there are obvious threats to US national security, such as China and Russia today, not devoting the preponderance of US defense resources and planning efforts to countering these threats would be imprudent.

This diversified approach to defense planning virtually ensures, whatever the next major demand for US military power will be, the US armed forces will not be maximally prepared to respond to it. This approach also ensures, however, no matter the contingency the US military faces, it will have at least some capability to respond. This initial capability, moreover, can form the basis for a rapid expansion, such as occurred with US special operations forces in the years following 9/11.

The terrorist attacks of 9/11 and developments over the subsequent two decades have shown, though certain contingencies and adversaries can be anticipated, the US armed forces will not be able to choose the next conflict in which they will be involved. As a result, the US military must prepare both for the contingencies that are predictable and those that seem unlikely but are nonetheless conceivable. This lesson is perhaps the most important one US defense planners should derive from the events of September 11, 2001.

  1. Office of the Secretary of Defense, Quadrennial Defense Review Report (Washington, DC: Office of the Secretary of Defense, September 30, 2001), 2n1.
  2. “Casualty Status,” Department of Defense, August 31, 2021,
  3. Neta C. Crawford and Catherine Lutz, “Human Cost of Post-9/11 Wars: Direct War Deaths in Major War Zones,” Watson Institute for International and Public Affairs, November 13, 2019,
  4. Julia McQuaid et al., Independent Assessment of US Government Efforts against Al-Qaeda (Arlington, VA: CNA, October 2017), iv.
  5. International Comparison Program, World Development Indicators, and Eurostat-Organisation for Economic Co-operation and Development Purchasing Power Parties Programme, “GDP, PPP (Constant 2017 International $),” World Bank, n.d.,; and Stockholm International Peace Research Institute, Military Expenditure by Country, in Constant (2019) US$ m., 1988–2020 (Stockholm: Stockholm International Peace Research Institute, 2021).
  6. Erin Miller, American Deaths in Terrorist Attacks, 1995–2019 (College Park, MD: National Consortium for the Study of Terrorism and Responses to Terrorism, 2020).
  7. Miller, American Deaths.

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