Twenty-five years after the Sept. 11, 2001 attacks, their consequences continue to shape global politics, security policy and conflicts from the Middle East to South Asia.
The attacks, carried out by 19 al-Qaeda hijackers using four passenger planes, killed 2,977 people at the World Trade Center in New York, the Pentagon near Washington, and near Shanksville, Pennsylvania.
They triggered the US-led “war on terror,” NATO’s first use of its collective defense clause, the invasion of Afghanistan, the creation of new US security institutions, the Guantanamo detention system, the Iraq war, the rise of new militant movements, and two decades later, the Taliban’s return to power.
The global fallout of the post-9/11 era extended far beyond initial battlegrounds. According to data from Brown University’s Costs of War project, post-9/11 conflicts have caused an estimated more than 900,000 direct deaths, including over 400,000 civilians, while the US federal government spent or obligated over $8 trillion through 2021.
Beyond direct combat, an estimated 3.6 to 3.8 million people have died indirectly from war-related destruction, disease, malnutrition, and infrastructure collapse across conflict zones in Afghanistan, Pakistan, Iraq, Syria, and Yemen.
Here is a timeline of key events from 2001 to the 25th anniversary.
Four hijacked planes were used in coordinated attacks on the US.
American Airlines Flight 11 struck the North Tower of the World Trade Center at 8.46 am local time, followed by United Airlines Flight 175 hitting the South Tower at 9.03 am.
American Airlines Flight 77 hit the Pentagon at 9.37 am, while United Airlines Flight 93 crashed near Shanksville, Pennsylvania at 10.03 am, after passengers and crew fought the hijackers.
The attacks became the deadliest terrorist assault on US soil and transformed Washington’s security and foreign policy.
One day after the attacks, NATO invoked Article 5 of its founding treaty for the first time in its history.
The clause treats an attack on one member as an attack on all. NATO later confirmed that the attacks were directed from abroad, making Article 5 fully operative.
The decision internationalized the US response and laid the basis for allied support in the Afghanistan campaign.
The US Congress passed the Authorization for Use of Military Force resolution, giving the president broad authority to use force against those responsible for the Sept. 11 attacks and those who aided them.
The authorization became the legal foundation for US counterterrorism operations across multiple countries over the following decades.
The US and Britain launched Operation Enduring Freedom against al-Qaeda and the Taliban in Afghanistan.
Washington said the Taliban had given safe haven to Osama bin Laden and al-Qaeda. The campaign quickly removed the Taliban from Kabul, but the war became the longest in US history, lasting nearly 20 years.
President George W. Bush signed the USA Patriot Act, expanding law enforcement and intelligence powers after the attacks.
Supporters said it gave authorities tools to detect and prevent terrorism. Critics said it weakened privacy protections, widened surveillance and gave the government excessive power in the name of national security.
The first detainees from Afghanistan arrived at the US naval base at Guantanamo Bay, Cuba.
The detention facility became one of the most controversial symbols of the war on terror, criticized over indefinite detention, interrogation practices and the use of military commissions instead of regular civilian courts.
More than two decades later, the prison remains open, despite repeated promises by US administrations to close it.
Bush signed the Homeland Security Act, creating the US Department of Homeland Security.
The new department brought together 22 federal agencies in one of the largest reorganizations of the US government in decades.
Its creation reflected how deeply 9/11 changed domestic governance, from airport screening and border security to emergency response and intelligence coordination.
The US and its allies launched a war against Iraq, arguing that Saddam Hussein’s government possessed weapons of mass destruction and had violated UN resolutions.
No active WMD stockpiles were found. The invasion toppled Saddam but unleashed years of insurgency, sectarian violence and instability.
The Iraq war became one of the most divisive consequences of the post-9/11 era and helped create conditions later exploited by extremist groups.
Ten bombs exploded on four commuter trains in Madrid during the morning rush hour, killing 191 people and injuring more than 2,000.
The attacks were carried out by militants inspired by al-Qaeda and came three days before Spain’s general election.
They showed that the post-9/11 threat had moved beyond the US and Afghanistan, striking major European cities and reshaping security debates across the continent.
Four suicide bombers attacked London’s transport network, killing 52 people and injuring more than 770.
Three bombs exploded on Underground trains and a fourth on a bus in Tavistock Square.
The attacks intensified debate in Britain and Europe about radicalization, intelligence failures, surveillance, civil liberties and the role of foreign policy in domestic security.
US special forces killed al-Qaeda leader Osama bin Laden in Abbottabad, Pakistan, nearly 10 years after the 2001 attacks.
His death closed a central chapter in the US pursuit of al-Qaeda’s leadership but did not end the wider conflict.
By then, al-Qaeda had developed regional branches, while the wars in Afghanistan and Iraq had already reshaped the political and militant landscape across the Middle East and South Asia.
The ISIS/Daesh terror group declared a “caliphate” across territory it controlled in Iraq and Syria, with Abu Bakr al-Baghdadi as its leader.
ISIS/Daesh grew out of al-Qaeda’s Iraq branch but split from its central leadership and built a territorial project far more visible and brutal than earlier terrorist networks.
Its rise marked a new phase in the post-9/11 era: mass recruitment through social media, foreign fighter flows, attacks across several continents and a US-led military campaign to destroy its territorial control.
The US and the Taliban signed an agreement in Doha setting terms for the withdrawal of US and allied forces from Afghanistan.
The deal excluded the Afghan government from direct negotiations and committed the Taliban to prevent groups including al-Qaeda from using Afghan soil to threaten the US and its allies.
The Taliban entered Kabul as the internationally backed Afghan government collapsed and President Ashraf Ghani fled the country.
Images of desperate evacuations at Kabul airport became one of the defining moments of the end of the US-led war.
The Pentagon announced the completion of the US military withdrawal from Afghanistan, ending America’s longest war.
The exit came just days after a suicide bombing near Kabul airport killed scores of Afghan civilians and 13 US service members.
A US drone strike in Kabul killed Ayman al-Zawahiri, bin Laden’s longtime deputy and successor as al-Qaeda leader.
UN monitoring reports later described Egyptian militant Saif al-Adel as al-Qaeda’s de facto leader, while also noting questions over his authority and location.
The 25th anniversary comes with the physical legacy of the attacks still present in New York, Washington and Pennsylvania, and the political legacy visible across the world.
The wars launched after 9/11 have formally ended or changed shape, but their consequences remain: expanded surveillance powers, transformed airport security, conflicts over detention and torture, fractured states, displaced populations, and a global counterterrorism architecture that continues to evolve.
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