Here's a look in pictures at the 20 years since 9/11, charting the subsequent "war on terror" and its wider impact on world events.
Coordinated plane attacks on the Twin Towers in New York and the Pentagon claim nearly 3,000 lives. It is the worst terrorist attack in history.
The response from the US, under the helm of President George W. Bush, was swift. In October the first strikes against Afghanistan were launched to "prevent the country from remaining a haven for Al Qaeda's Islamist terrorists".
A year later, Osama bin Laden claimed responsibility for the attacks on behalf of Al Qaeda.
The Taliban, who had condemned the attacks but refused to hand over bin Laden, put up little resistance.
2002: Concerns about the war in Afghanistan arise
The US government opens the Guantanamo Bay detention centre in January to detain and interrogate suspected Islamist terrorists.
The Naval base is located in Cuba and Washington is accused of disregarding the rule of law and subjecting detainees to torture and abuse.
Nearly 800 people will be detained there overall. Today, 39 remain.
In Afghanistan, the Taliban government falls within a few months and is then succeeded by a series of interim governments.
2003: Iraq is invaded
American troops invade Iraq in March as part of its "War on Terror". The Bush administration had accused the country's leader Saddam Hussein of being one of the world's "sponsors of terrorism" and being part of the famous "Axis of Evil" alongside Iran and North Korea.
Hussein was also accused of harbouring terrorists and the coalition said the invasion aimed to rid the country of alleged weapons of mass destruction.
2004: Islamist terrorism strikes in Europe
Brutal attacks on commuter trains in Madrid on March 11, 2004, kill nearly 200 people. It is the first of a wave of deadly Islamist terrorist attacks across the Old Continent, which also include London and Manchester in 2005 and 2017, Paris and Nice in 2015 and 2016, and Berlin, also in 2016.
Islamist terrorism also scarred Istanbul, Bali, Mumbai and Colombo.
2005: Afghanistan proves difficult
Despite the relative success of the campaign in Afghanistan to oust the Taliban, negotiations with the country's tribal leaders prove difficult. Elections are held four years after the invasion but attacks continue on a daily basis.
2006: Saddam Hussein executed
On December 20, deposed Iraqi leader Saddam Hussein is convicted of crimes against humanity by the prosecution of his country's interim government and is sentenced to death. He is killed by hanging 10 days later.
2007: 500 killed in Iraq attack
Ending the previous regime did not end tensions in Iraq. On August 19, nearly 500 people are killed in Qahtaniya near Mosul in a truck bomb attack. The victims belonged to the Yazidi community, a Kurdish sect considered blasphemous by Islamists. The US blames Al Qaeda for the attack.
2008: US eyes Afghanistan withdrawal
The training of Afghan forces by the coalition is accelerated with the aim to withdraw by 2014.
2009: Evidence of abuses committed by international troops
The Blackwater trial ends with employees of the private security firm contracted by the US found to have killed Afghan civilians.
2010: WikiLeaks embarrasses the US
WikiLeaks reveals details of "collateral murders" committed by the US in Iraq through the publications of thousands of confidential diplomatic cables. Private Chelsea Manning is arrested for allegedly being one of the sources of the biggest leak in history.
Furthermore, WikiLeaks' Afghan "War Diaries" detail "collateral" civilian deaths, kidnappings, torture and Pakistan's participation in the defence of the Taliban.
2011: Bin Laden captured
On May 2, US President Barack Obama announces that Osama Bin Laden had been captured and killed.
Washington's withdrawal from Iraq is completed in December. Terrorist groups take advantage to proliferate including the so-called Islamic State.
2012: Julian Assange enters Ecuadorian embassy
Pursued by the US justice system for treason and espionage, WikiLeaks founder Julian Assange seeks refuge in the Ecuadorian embassy in London.
He will stay there for seven years during which time WikiLeaks continues to public compromising documents for governments and companies.
Assange is arrested by British police in April 2019 upon exiting the embassy after losing the support of the new Ecuadorian government.
Judges have so far refused to extradite him to the US because of his psychological state.
2013: Edward Snowden's revelations
Documents leaked by Edward Snowden, a former employee of the US National Security Agency (NSA), reveal the existence of a vast Internet and phone surveillance programme run by US intelligence.
The programme was initially set up to counter terrorist threats but the documents suggest that it was soon extended to spy on the leaders of rival or allied countries as well as individuals and entities that posed no threat to the US.
2014: The Islamic State claims a caliphate
Sectarian violence in Syria and Iraq enables the extremist group to recruit, grow and conquer new territory across both countries. IS also forges alliances with other international terrorist groups such as Nigeria's Boko Haram.
2015: Europe's migrant crisis
The flow of migrants and refugees from Syria, by then four years into a devastating civil war, continues unabated. The shocking image of Aylan Kurdi, a toddler washed up dead on the tourist beaches of Bodrum, awakens Europe to the drama unfolding in its own backyard.
The impact of this tragedy prompts several campaigns for European governments to welcome refugees and Chancellor Angela Merkel keeps Germany's borders open. More than a million people will claim asylum in Germany that year.
2016: Turkey and EU strikes migrant deal
Ankara and Brussels reach an agreement to stem the flow of migrants reaching the European Union. Turkish President Recep Tayyip Erdogan agrees to increase border security and take back irregular entries into Greece in exchange for €6 billion.
The migrant crisis has meanwhile impacted Europe's politics with far-right parties throughout the 27-country bloc gaining momentum including Alternative for Germany, and the League in Italy.
2017: The migrant nightmare in Libya
As Europe and Turkey tighten entry conditions at their borders, the flow of migrants trying to reach Europe via Libya, increases. Italy signs a new memorandum with Libya, paying Libyan coastguards to intercept boats departing their shores for Europe.
NGOs raise the alarm on the detention conditions intercepted migrants are subjected to while they await voluntry or forced return to their countries of origin.
2018: Afghan withdrawal back on the table
Western leaders discuss a new timetable for the withdrawal of their troops from Afghanistan at a NATO summit in July. US President Donald Trump, who had criticised the US engagement in Iraq and Afghanistan, during his campaign, wants all troops out before the 20th anniversary of the 9/11 attacks.
2019: IS leader killed
IS leader Abu Bakr al-Baghdadi is killed in Barisha, Syria in October in a drone operation. The US Department of Defence provides full details of the operation including infrared images of the drone that destroyed the house.
2020: US negotiates with the Taliban
The Trump administration negotiates the US withdrawal from Afghanistan with the Taliban militant group before September 2021. The initial deal plans for US troops to be out by May and calls on the Taliban to prevent Afghanistan from becoming a new haven for terrorist groups.
2021: Taliban takeover
The Taliban launch a breakneck campaign to reclaim Afghanistan as foreign troops pull out. The group's campaign leaves Western powers scrambling to evacuate their nationals and Afghans who worked for them over fears for their safety. The withdrawal is completed on August 31 with US troops ceding control of the Kabul airport.
The Taliban promise that things will be different from their previous regime in the late 1990s, declaring a general amnesty and announcing that women will be allowed to work. Its interim government announced in the days preceding September 11, is stacked with veterans of its previous hardline regime. Fighters also violently suppress demonstrations, including for women's rights.