Twenty-one years on, and the September 11 attacks have continued to send ripples across the world.
While a lot of focus is rightly put on the iconic Twin Towers (where nearly 3,000 people were killed after al-Qaeda terrorists hijacked two passenger jets and flew them into the buildings, causing their collapse, a total of four planes were involved that day.
A third flight - American Airlines Flight 77 - also came to a tragic end, crashing into the side of the Pentagon building near Washington DC.
Here is everything you need to know about it.
On the morning of Tuesday 11 September 2001, American Airlines Flight 77 was scheduled to depart the Washington Dulles International Airport in Virginia, on its way to Los Angeles International Airport.
But less than 35 minutes into the flight - shortly after American Airlines Flight 11 struck the North Tower of the World Trade Centre - the Boeing 757-223 aircraft was hijacked by five men affiliated with al-Qaeda, who stormed the cockpit and forced the passengers, crew, and pilots to the rear of the aircraft.
Hijacker Hani Hanjour - who had trained as a commercial pilot at the CRM Airline Training Center in Arizona in 1999 - assumed control of the flight.
Normal radio communications to air traffic control ceased, and a few minutes later the plane began to deviate from its normal assigned flight path, turning south.
The plane’s transponder was then switched off as the hijackers set the flight’s autopilot on a course heading east towards Washington, DC.
Unbeknownst to the hijackers, passengers aboard made telephone calls to friends and family and relayed information on the hijacking.
Air traffic controllers asked a passing fighter jet to follow the aircraft, but the pilot said he had difficulty picking out the airplane in the "East Coast haze". He then saw a "huge" fireball and assumed the aircraft had hit the ground.
An hour and 17 minutes after takeoff - at 9.37am - the plane was deliberately crashed into the western side of the Pentagon, the impressive five-sided structure that serves as the headquarters for America’s military.
While level above the ground and seconds from impact, the Boeing’s wings clipped five street lamp posts, before impacting the Pentagon at the first-floor level at a speed of 530mph.
The airplane took eight-tenths of a second to fully penetrate 94 m into the three outermost of the building’s five rings.
The impact severely damaged an area of the Pentagon and caused a large fire. A portion of the building later collapsed, and firefighters spent days working to fully extinguish the blaze.
How many people died?
All 64 aboard the aircraft - including six crew and the five hijackers - died, as well as another 125 personnel inside the building, taking the death toll to 189.
At the time of the attacks, approximately 18,000 people worked at the Pentagon; the section that was struck housed the Naval Command Centre.
The fatalities included 55 military personnel and 70 civilians. Of those 125 killed, 92 were on the first floor, 31 were on the second floor and two were on the third.
Is there a memorial?
The damaged sections of the Pentagon were rebuilt in 2002, with occupants moving back into the completed areas that August.
The Victims of Terrorist Attack on the Pentagon Memorial was opened at the nearby Arlington National Cemetery in 2002, and specifically honours the five individuals from which no identifiable remains were found, including three year old Dana Falkenberg.
A portion of the remains of 25 other victims are also buried at the site.
The 184 victims of the attack are also memorialised in the Pentagon Memorial adjacent to the crash site, a 1.93-acre park that contains a bench for each of them arranged according to their year of birth.
At the National September 11 Memorial in New York City - which commemorates the attacks in the exact spots the Twin Towers once stood - the names of the Pentagon victims are inscribed on six panels at the South Pool.
A message from the editor:
Thank you for reading. NationalWorld is a new national news brand, produced by a team of journalists, editors, video producers and designers who live and work across the UK. Find out more about who’s who in the team, and our editorial values. We want to start a community among our readers, so please follow us on Facebook, Twitter and Instagram, and keep the conversation going. You can also sign up to our newsletters and get a curated selection of our best reads to your inbox every day.