The 9/11 attacks killed thousands, traumatized a nation and changed the course of American history — but also triggered a chain of events that dramatically altered millions of lives half a world away.
The U.S. has waged a war on terror in Afghanistan and Iraq while also hunting down Osama bin Laden in Pakistan.
NBC News spoke to teens in those three countries to show the impact through their eyes — which have seen nothing but a post-9/11 world.
Hassan Madadi was only four months old when planes hit New York's twin towers and the Pentagon, killing some 3,000 people.
He was too young to remember how a U.S.-led coalition invaded his country only weeks later to topple the Taliban, which had sheltered Bin Laden. But the aspiring writer is grateful.
"If the September 11 attacks did not happen and the United States had not come to Afghanistan, most likely I would have been sitting at a religious madrasa reciting verses of the Quran," he told NBC News. "I I would never have had the opportunity to get the education I am getting today ... the Taliban would not allow any other books except for religious texts."
The Taliban had brutally imposed its strict interpretation of Islam on a population already scarred by decades of conflict.
Life under the Taliban had been particularly hard for Madadi and his family, members of the long-oppressed Hazara minority who live in Kabul. The Hazara are predominantly Shiite Muslim — reviled as apostates by fundamentalist Sunnis such as the Taliban.
But freed from the constraints of the Taliban, Madadi was able to pursue his passion: Literature.
"I was introduced to books by my classmate Mohammad when I was 10," he explained. "I convinced my mom to pay library fees … This opened my way to books and ever since I read at least one book every week."
Madadi is mindful of the contrast between his childhood and his parents': His father got only a third-grade education; his mother never went to school.
"I have big plans for my future, I want to go to literature faculty after finishing school and get my master's degree," he told NBC News.
While he doesn't hesitate to dream, the prospect of losing that future is never far from his mind.
"I always have this fear and worry that all that we have today could be taken from us and we have Taliban come back," he said.
That's because, despite the billions of aid and security assistance that poured into Afghanistan since the invasion, record numbers of Afghan civilians are being killed or maimed by violence. The Taliban today controls more territory now than since it was toppled — and is encroaching on Kabul.
The violence has prevented some 40 percent of Afghan children from going to school. Madadi is one of the lucky ones to get an education — for now.
"My friends and I live in a state fear because we're always watching news about the war and districts falling to the Taliban," he said. "I also fear losing what we have, not just our home, but the opportunities we have — our school, our city and our way of life."
While the anxiety is constant with him, youthful optimism is not far behind.
"The Taliban will never be able to enforce their backward ideas on people," he said. "Even if they manage to take over all of the countries, the people will rise against them — the new generation's awareness is much higher and will fight for the way of life they have been brought up with."
When President George W. Bush's administration set its sights on Iraq, Ruqaya Ali's family had every reason to be optimistic.
Saddam Hussein's brutal regime had persecuted the teen's father and executed her uncle — one of some 250,000 Iraqis believed to have been killed during the dictator's 25-year reign.
When the U.S. ultimately invaded in 2003 and toppled Saddam, Ali's father was hopeful for positive change.
"My father told me there was an attack on America one month before I was born, and because of that the U.S. decided to get rid of Saddam Hussein," the aspiring dentist said, explaining the course of events. "Saddam was a part of an attack there, and Saddam was killing Iraqi people."
The reality was more complicated: The U.S. contended Saddam was hiding weapons of mass destruction.
History showed that calculus to be wrong — along with the misplaced dreams of Ali's family.
The U.S.-led invasion plunged Iraq into a spiral of sectarian violence between Sunni and Shiite Muslims that claimed an estimated 165,000 lives.
Amid the worst of the violence in 2006, Ali and her Sunni family fled their home in the predominantly Shiite Muallimeen area of southwest Baghdad. They were scared of being killed by Shiites, Ali explained:
"We just got out with bags filled with clothes," she said, recounting stories her parents have told her.
They were among what the United Nations estimates were 370,000 civilians displaced by the violence.
While that spate of bloodshed abated, today Ali's family is confronting a new threat: ISIS.
The teen has managed to maintain a sense of normalcy despite the violence again sweeping her country, with high hopes for her future as a dentist.
"I am a talented girl. I got good grades last year and want to keep them up this year," she told NBC News.
That doesn't mean she isn't aware of the terror group — which Ali condemns as "criminals."
She's also come to her own conclusions about the outside intervention her father once longed for.
Before the U.S. arrived, "there were no explosions, no killings, no kidnapping," Ali said.
It was only after the foreign troops came that "Sunnis and Shiites began killing each other, even if they were friends or neighbors who lived side-by-side for years," she added.
Azhar Mahood feels like he and his fellow Pakistanis are being punished for a crime they didn't commit.
He wasn't even born when the twin towers and Pentagon were hit. No Pakistanis were implicated in the attack.
"I don't understand why we are paying for the terrorist attacks in the U.S.," the 14-year-old told NBC News.
The invasion of Afghanistan drove militants and fighters across a porous neighboring border with Pakistan, where they've waged an insurgency under the mantle of the Pakistani Taliban.
The government estimates some 23,000 people have been killed since 2002 — with civilians suffering the most — and launched a series of offensives to root out the militants, to mixed reviews.
Mahmood was born in Miranshah, the main city in the mountainous North Waziristan region home to much of the militant spillover.
He was only eight when a suicide bomber detonated in front of him at a school.
"There was a huge explosion in the parking area where dozens of cars had arrived to pick up the students," he said. "There was a thick black smoke all around us. Then loud firing by the security forces."
While unhurt, Mahmood said he was terrified and stayed home from school for two weeks after the Taliban attack. But he eventually had to return to classes.
"It was a difficult decision for our family because some of the family wanted me and my cousins to continue our studies my hometown, while my father and some family members wanted to send us to away to Peshawar," he said, referring to the provincial capital some 150 miles away.
School administrators and local officials struggled to keep the kids safe, changing dress codes and drop-off routines while stepping up security.
But a 2009 mortar attack that hit his school changed everything. His father later moved the family to Peshawar.
Mahmood had trouble adjusting to his new life and has had difficulties making friends.
He frequently remembers classmates who also fled the violence.
"I' still haven't seen or heard from my friends Zubair, Israr, and Awais," Mahmood said. "I wish I could visit my hometown and see my house."
Even the relative safety his family sought in Peshawar has been shattered: A December 2014 attack on the school killed 141 people, mostly students.
While Miranshah is no safer, Mahmood longs for it. He repeatedly asks his father if they'll ever return home.
And Sher Bahadur Mahmood repeatedly tries to reassure him, saying repeatedly, "Sure we will."