Few events in history left such drastic consequences as those in New York and Washington when the World Trade Centre and Pentagon buildings were hit by hijacked passenger aircraft. And although most people can readily recall where they were when the world changed forever, those who served in the Armed Forces would have their lives impacted starkly.
George W. Bush vowed to seek revenge within hours of the attacks, telling the US, "our military is powerful, and it is prepared."
The President meant every word.
As the attacks played out, newscasters rolled continuous coverage, sights the likes of which had never been seen on television.
For troops on duty that day, routine activities halted as everyone looked on in horror.
"I remember the exact place I was stood down to a millimetre when the second plane hit that tower."
Shaun Goater was a 29-year-old Corporal of Horse based at Combermere Barracks, Windsor, in September 2001. He recalled the scenes there as the chaos stateside played out:
"We were expecting to attend a lunchtime mess meeting, and we were making our way across to the mess. I walked in and could see people standing around in shock.
"When I saw what was unfolding on the TV, I couldn't believe what was happening.
"This was just after the first plane had hit, and the news was just breaking on British TV. No one really knew what was actually happening. It wasn't until we watched the second plane hit that it became clear that the world was under attack.
"When the second plane hit, the mess erupted in cries of disbelief and then shocked silence as the enormity of what was happening hit home.
"Everyone was just so enthralled & shocked by what we were watching."
Another soldier, Adam Kemp, was returning to phase two training after a long weekend in Newcastle and witnessed the reaction to the events while travelling on a train to Kings Cross. He remembered the reactions of another passenger becoming aware of the attacks. Adam said:
"The man was sat alone behind me muttering the words' oh my god' and it was clear whatever he was looking at or hearing about was a very serious situation. He asked me if I had seen what was happening in New York? This was the first that I heard or knew about the attacks.
"I called my wife Lucy and asked her to put on the news. She confirmed that passenger planes had crashed into the World Trade Centre. With something as surreal as that, the severity of what you are hearing doesn't quite register.
"I remember not connecting the possibility of an attack in Central London which was where my train was heading, until my wife said she didn't want me travelling into London. I told her not to be silly, but it was on the news that Canary Wharf was being evacuated about an hour after the attack. That's when it really kicked in that I was heading to a city that was expected some sort of terrorist attack.
"When I arrived in Kings Cross, the place was awash with businessmen and women all hurrying about trying to get a train out of the city. The Tube was packed, and everyone had worried-looking faces."
Back at Windsor, some of Shaun's colleagues in the mess had already begun discussing what the consequences of the news might be on life in the military. He remembered:
"After we had been watching for about an hour, the RCM [Regimental Corporal Major] stood up and said something along the lines of 'Right, this is obviously not the time to be holding a pointless mess meeting.
"'What we have just watched changes everything.'
"We all knew at that point we would be going to war. We didn't know where exactly, but we knew we would be going away and soon."
Like Adam, Shaun's thoughts turned to his loved ones: "We were expected to go back to work, I know most of us who were married just wanted to go home to be close to our families such was the enormity of what we had just witnessed."
That enormity had military consequences both immediately and in the more drawn-out sense of the operations that would follow, which both Shaun and Adam would be deployed upon repeatedly for the following decade. Adam recalled the heightened state of affairs as he arrived back at base a short while after getting to London. He said:
"By the time I had reached camp, the barracks was on a much higher state of alertness. The guys on the gate had webbing and helmets on with rifles which were strange to see.
"I remember there being a tremendous sense of what happens next. The whole world was in shock, and we were about to be dragged into the next phase of whatever was happening.
"There was a strange sense of the likelihood of this causing a war, but confusion as to how it would play out. Who would be called to fight? Would the regiment be deployed? What would happen to us as new recruits?
"Would we be expected to deploy? It might seem silly now, but at that moment there was just so much shock."
Those sentiments of uncertainty, particularly from the perspective of fighting wars, were echoed by Shaun. He had already seen multiple operations in theatres worldwide by that stage in his career. He said:
"We had spent the previous decade serving in Bosnia and the Former Yugoslavia Republic, where we were on peacekeeping missions. We all knew this next operation would be a kinetic one, which obviously affected people and their families.
"My eldest daughter was only a few months old at the time and I remember going home and holding her extra tight as I didn't know how long I would get to spend with her before going to whichever theatre we were going to.
"The thought of us not going somewhere never entered our heads especially seeing as our role would definitely be one of the first to be deployed. It turned out that only D Squadron, HCR, went on the initial deployment when it came, but at the time, we had no idea of the numbers required.
"My eldest daughter turned 20 earlier this year and that milestone brought back memories of the 9/11 attacks."
Both men served on operations in both Iraq and Afghanistan as a ramification of what occurred that day.
Afghanistan has been back in the news recently following the withdrawal of NATO forces and the knock-on effect of the reemergence of a Taliban government. Adam was sombre about how matters have played out and reflected on colleagues he lost on those operations. He said:
"The withdrawal was always going to result in the Taliban moving back in and retaking power. If I could see this then I'm pretty sure the world leaders and big decision makers could see it, too. I think although inevitable, the actual withdrawal was masterminded by an incompetent US President who either acted on impulse to point score or he was very poorly advised."
Shaun was more upbeat on matters and reflected on some positives from the long operations he and his friends completed. He said:
"We did some great work in both Iraq and Afghanistan, and I truly believe we made things better for the populations of both countries. Did we do enough? Probably not, but I think that was down to the political will waning as the years passed.
"I often think of those we lost, and when I returned from Afghanistan, I admit that I struggled to cope and made things very hard for my family. It was only through their love and support that I managed to get control of my emotions and get back to some sort of normal life.
"Those of us that returned were the lucky ones. Seeing the damage this withdrawal has done has only served to make me ask, was it worth it?"
For Adam, anniversaries of the terror attacks bring emotions to the surface. He said:
"Anniversaries of 9/11 cause me anxiety. Each year since there's been that question about will they mark it with another attack. I remember booking a holiday one year, and the flights were on September 11, so we changed the dates.
"Sometimes I forget that it was that attack that triggered it all, because for pretty much my whole career, Iraq or Afghanistan was my sole focus. It just became the norm, and perhaps the reasons for those operations became a bit of a distant memory."
Shaun compared those moments in the mess that day with other historical events. He remarked that of all the experiences he had in the military and his life in general, seeing the towers fall was the one memory that stood out above all.
"I was on duty in Horse Guards when the Downing Street mortar attacks took place in 1991," Shaun said.
"9/11 was 100 times worse. It is hard to explain just how significant those events were. People who were alive when Kennedy was killed will always know where they were at that time. 9/11 is the same - if you can't remember where you were, then you weren't there."
Cover: World Trade Center, New York City terrorist attack, 11 September 2001. Alamy.