This article traces the history of the United States' War on Terror from the September 11, 2001 attacks through the withdrawal from Afghanistan in 2021 and its aftermath. For broader context, see United States - History. For interpretive debates, see United States War on Terror - Debate.

On the morning of 11 September 2001, nineteen hijackers affiliated with the Islamist militant organization al-Qaeda seized four commercial aircraft. Two planes were flown into the World Trade Center towers in New York City, one into the Pentagon in Arlington, Virginia, and one crashed into a field near Shanksville, Pennsylvania after passengers attempted to overpower the hijackers. Approximately 2,977 people were killed in the attacks, making it the deadliest terrorist attack on United States soil in the nation's history.

President George W. Bush addressed the nation that evening, characterizing the attacks as acts of war. Within days, the administration identified al-Qaeda and its leader Osama bin Laden, then sheltering in Afghanistan under the Taliban government, as responsible. On 18 September 2001, Congress passed the Authorization for Use of Military Force (AUMF), authorizing the president to use force against those who planned, authorized, committed, or aided the attacks. The legislation passed with near-unanimous support; Representative Barbara Lee of California cast the sole dissenting vote in either chamber.

On 20 September 2001, Bush addressed a joint session of Congress and formally announced what he termed the “War on Terror,” declaring it a struggle that would not end with a single battle or against a single organization. He issued an ultimatum to the Taliban government of Afghanistan to surrender bin Laden and dismantle al-Qaeda training camps.

After the Taliban rejected the ultimatum, the United States and allied forces launched Operation Enduring Freedom on 7 October 2001. Coalition air strikes targeted Taliban military assets and al-Qaeda training facilities. Ground operations were conducted initially in coordination with the Northern Alliance, an Afghan opposition coalition that had been fighting the Taliban since the 1990s.

The Taliban government collapsed rapidly. Mazar-i-Sharif fell on 9 November 2001, Kabul on 13 November, and Kandahar - the Taliban's stronghold - on 7 December. Bin Laden and senior al-Qaeda leadership were believed to have escaped into the Tora Bora mountain region along the Pakistani border in December 2001. A large-scale battle at Tora Bora failed to capture or kill bin Laden.

The Bonn Agreement of December 2001 established an interim Afghan administration under Hamid Karzai. The International Security Assistance Force (ISAF) was authorized by the United Nations Security Council in December 2001 to support the new government. A loya jirga (grand council) in June 2002 selected Karzai as transitional head of state. A new Afghan constitution was ratified in January 2004, and presidential elections were held in October 2004.

Taliban forces regrouped in Pakistan's tribal areas and began an insurgency against the Karzai government and coalition forces. Violence increased progressively from 2005 onward. NATO assumed command of ISAF in 2006 and expanded operations to southern and eastern Afghanistan.

President Barack Obama, who took office in January 2009, ordered a surge of approximately 30,000 additional troops to Afghanistan, bringing total U.S. force levels to roughly 100,000 by 2010. The surge aimed to reverse Taliban momentum and accelerate the training of Afghan security forces. Obama simultaneously announced that U.S. forces would begin withdrawing by July 2011.

Osama bin Laden was killed on 2 May 2011 in Abbottabad, Pakistan, in a raid by U.S. Navy SEALs acting on intelligence developed over several years. Pakistani authorities were not informed of the operation in advance.

The U.S. and NATO formally concluded their combat mission in Afghanistan on 28 December 2014, transitioning to a training and advisory role under Operation Resolute Support.

In the months following September 11, the Bush administration began making the case that Iraq under Saddam Hussein represented a threat requiring military action. Administration officials asserted that Iraq possessed weapons of mass destruction (WMD) and had links to terrorist organizations. Secretary of State Colin Powell presented the administration's case to the United Nations Security Council on 5 February 2003, citing intelligence assessments of Iraqi WMD programs.

Congress passed a separate AUMF authorizing force against Iraq on 16 October 2002. Coalition forces, led by the United States and the United Kingdom, invaded Iraq on 20 March 2003. Baghdad fell on 9 April 2003. Bush declared the end of major combat operations on 1 May 2003 in a speech aboard the USS Abraham Lincoln beneath a banner reading “Mission Accomplished.”

No stockpiles of WMD were found. The Iraq Survey Group, established to search for WMD after the invasion, concluded in its 2004 Duelfer Report that Iraq had ended its nuclear, chemical, and biological weapons programs in 1991 and had no active programs at the time of the invasion.

A Sunni-led insurgency and sectarian violence between Sunni and Shia communities escalated through 2004-2006. Abu Musab al-Zarqawi established al-Qaeda in Iraq (AQI), which carried out large-scale bombings including the 2006 destruction of the al-Askari mosque in Samarra, a major Shia holy site that precipitated intensified sectarian conflict. Zarqawi was killed in a U.S. airstrike in June 2006.

In January 2007, Bush announced a troop surge of approximately 20,000 additional soldiers. Violence decreased significantly through 2007 and 2008. The United States and Iraq signed a Status of Forces Agreement in November 2008 providing for the withdrawal of U.S. combat forces by 31 December 2011. The last U.S. combat troops departed Iraq in December 2011.

Following the September 11 attacks, the Bush administration developed legal frameworks for the detention and interrogation of captured combatants. Enemy combatants captured in Afghanistan and elsewhere were designated as unlawful combatants not entitled to prisoner-of-war protections under the Geneva Conventions, a determination contested by legal scholars and later courts.

The detention facility at Guantanamo Bay, Cuba, received its first detainees in January 2002. At its peak, it held approximately 780 detainees. Military commissions were established to try detainees, though their legal basis was challenged repeatedly in federal courts. The Supreme Court ruled in Hamdan v. Rumsfeld (2006) that the original military commissions lacked proper congressional authorization and that Common Article 3 of the Geneva Conventions applied to detainees. Congress subsequently passed the Military Commissions Act of 2006.

The CIA operated a program of enhanced interrogation techniques (EITs) authorized by Justice Department legal opinions - later known as the “torture memos” - drafted by John Yoo and Jay Bybee in 2002. Techniques included sleep deprivation, stress positions, and waterboarding. The program was conducted at secret facilities known as black sites in various countries. The Senate Select Committee on Intelligence released a summary of its investigation of the CIA detention and interrogation program in December 2014, concluding that the techniques were more brutal than reported to oversight authorities and that the program did not produce unique intelligence that prevented attacks.

The NSA's warrantless surveillance program, authorized by Bush following September 11 and first reported publicly by the New York Times in December 2005, collected telephone and internet communications data on a large scale. The program's legal basis, under the president's Article II authority and the September 2001 AUMF, was debated extensively. Congress passed the Foreign Intelligence Surveillance Act Amendments Act of 2008, providing a revised legal framework.

Al-Qaeda in Iraq reconstituted itself as the Islamic State of Iraq and Syria (ISIS, also known as ISIL or Daesh) under Abu Bakr al-Baghdadi. Exploiting the Syrian civil war that began in 2011, ISIS seized large territories in Syria and northwestern Iraq. In June 2014, ISIS captured Mosul, Iraq's second-largest city, and declared a caliphate. The rapid advance of ISIS forces prompted the Obama administration to authorize airstrikes in Iraq in August 2014 and in Syria in September 2014.

A U.S.-led coalition conducted an extended air campaign against ISIS, while ground operations were carried out primarily by Iraqi security forces, Kurdish Peshmerga, and the Syrian Democratic Forces (SDF). Mosul was retaken by Iraqi forces in July 2017 after a nine-month battle. Raqqa, ISIS's de facto capital in Syria, fell to SDF forces in October 2017. Al-Baghdadi was killed in a U.S. special operations raid in northwestern Syria on 26 October 2019.

The Trump administration negotiated the Doha Agreement with the Taliban on 29 February 2020, providing for a phased U.S. and NATO withdrawal in exchange for Taliban commitments not to harbor groups threatening the United States and to engage in intra-Afghan peace talks. U.S. force levels in Afghanistan were reduced to approximately 2,500 by January 2021.

President Joe Biden announced on 14 April 2021 that U.S. forces would withdraw unconditionally by 11 September 2021, later moved to 31 August 2021. The Taliban launched a rapid offensive as withdrawal proceeded. Afghan government forces collapsed rapidly. Kabul fell to the Taliban on 15 August 2021. The U.S. conducted a large-scale evacuation from Kabul's Hamid Karzai International Airport; an ISIS-K suicide bombing at the airport on 26 August 2021 killed 13 U.S. service members and more than 170 additional people. The last U.S. military aircraft departed Kabul on 30 August 2021, ending nearly 20 years of U.S. military presence in Afghanistan. The Taliban re-established control of the country.

• Whether the intelligence assessments of Iraqi WMD were the product of analytic failure, political pressure, or deliberate misrepresentation is disputed; see United States War on Terror - Iraq Intelligence Debate.
• Whether enhanced interrogation techniques used by the CIA constituted torture under domestic and international law, and whether they produced actionable intelligence, are contested questions; see United States War on Terror - Interrogation Debate.
• Whether the 2007 troop surge or the Sunni Awakening was the primary cause of reduced violence in Iraq is disputed among military historians and analysts; see United States War on Terror - Iraq Surge Debate.
• Whether the U.S. failure to capture bin Laden at Tora Bora in December 2001 resulted from insufficient force commitment, command failures, or reliance on Afghan allies is disputed; see United States War on Terror - Tora Bora Debate.
• Whether the pace and manner of the 2021 Afghanistan withdrawal was the cause of the Taliban's rapid reconquest, or whether collapse was inevitable regardless of withdrawal timeline, is a contested causal question; see United States War on Terror - Afghanistan Withdrawal Debate.
• Whether the broad AUMF of 2001 authorized the range of military and surveillance activities conducted under its authority is disputed among legal scholars; see United States War on Terror - AUMF Debate.

1. National Commission on Terrorist Attacks Upon the United States, The 9/11 Commission Report (W.W. Norton, 2004). Full text at govinfo.gov.
2. Authorization for Use of Military Force, Pub. L. 107-40, 115 Stat. 224, 18 September 2001. Text at congress.gov.
3. Bob Woodward, Bush at War (Simon & Schuster, 2002).
4. Ahmed Rashid, Descent into Chaos: The United States and the Failure of Nation Building in Pakistan, Afghanistan, and Central Asia (Viking, 2008).
5. Charles Duelfer, Comprehensive Report of the Special Advisor to the DCI on Iraq's WMD (Iraq Survey Group, 2004). Available at cia.gov.
6. Thomas E. Ricks, Fiasco: The American Military Adventure in Iraq (Penguin Press, 2006).
7. Thomas E. Ricks, The Gamble: General David Petraeus and the American Military Adventure in Iraq, 2006-2008 (Penguin Press, 2009).
8. Senate Select Committee on Intelligence, Committee Study of the Central Intelligence Agency's Detention and Interrogation Program (Executive Summary, declassified December 2014). Available at intelligence.senate.gov.
9. Hamdan v. Rumsfeld, 548 U.S. 557 (2006). Full text at supremecourt.gov.
10. Military Commissions Act of 2006, Pub. L. 109-366.
11. Jessica D. Lewis, The Islamic State: A Counter-Strategy for a Counter-State (Institute for the Study of War, 2014).
12. Agreement for Bringing Peace to Afghanistan between the Islamic Emirate of Afghanistan and the United States of America, 29 February 2020 (Doha Agreement). Text at state.gov.
13. Special Inspector General for Afghanistan Reconstruction (SIGAR), What We Need to Learn: Lessons from Twenty Years of Afghanistan Reconstruction (August 2021). Available at sigar.mil.