Anthony Sadler, Spencer Stone and Alek Skarlatos were aboard a high-speed train en route to Paris from Amsterdam on Friday when a gunman opened fire.
Along with two others -- a French national and a Briton -- they charged, tackled and subdued him, officials said.
Applause poured in from world leaders and citizens alike as more details emerged on the chaotic, bloody scene aboard the train.
The 22-year-old Skarlatos, a National Guardsman based in Oregon, was on a monthlong vacation after his return from deployment in Afghanistan.
But instead of taking in scenic European countrysides, he was taking on an attacker aboard a zipping train.
Skarlatos heard gunfire and breaking glass, and sensed something was wrong. When the suspect appeared with a gun in their car, he mobilized others.
He is the one who yelled "get him" after he heard a gunshot and noticed that the gunman's weapon appeared to be jammed.
"The guy had a lot of ammo," Skarlatos said of the attacker. "His intentions were pretty clear."
Once they charged at him, he seized the gunman's rifle and pounded him in the head with the muzzle repeatedly.
"I'm so proud of (my son). I mean I'm in awe," his father, Emanuel Skarlatos, told CNN affiliate KVAL
in Roseburg, Oregon. "He inspires me."
Sadler, a senior at California State University in Sacramento, was on his first trip to Europe when terror struck.
When his friends jumped the gunman and took him down, he and another passenger helped restrain him and ensure he stayed down.
"The three of us beat up the guy," Sadler said. "In the process, Spencer gets slashed multiple times by the box cutter, and Alek takes the AK away."
Sadler helped tie up the suspect then went to other train cars to reassure passengers, said Jean-Hugues Anglade, a French actor who was traveling with his family.
He "came running into our car, yelling that the shooter was overpowered by American soldiers on leave, that everything was fine," Anglade told French magazine Paris Match. "He reassured us, he looked for survival blankets and a first aid kit for two seriously injured people."
It'll be hard to top his first trip to Europe and his final year in college.
"I'm just a college student, it's my last year in college. I came to see my friends on my first trip in Europe and we stopped a terrorist, it's kind of crazy," Sadler told CNN.
He urged people to be proactive during such situations.
"The gunman would have been successful if my friends had not gotten up," he said.
Briton Chris Norman rushed to help the Americans overpower the suspected gunman.
"All of us were in the same carriage, these two guys plus Spencer -- the guy who got injured -- were sitting on the same seats, the same row at the back of the train," Norman said.
Norman, who said he was sitting at the front of the car, saw a train employee dash past.
"I looked up, I saw a guy carrying an AK-47, or at least I assumed it was some kind of machine gun anyway," he said. "I ducked down in my seat."
When Skarlatos and his friends tackled the gunman, Norman said rapid reasoning" prompted him to jump in and help subdue the gunman.
"My thought was, 'OK, I'm probably going to die anyway, so let's go.' I'd rather die being active, trying to get him down, than simply sit in the corner and be shot. Either you sit down and you die or you get up and you die. It was really nothing more than that."
He said his instincts kicked in.
"We've seen enough of these kinds of attacks to understand that they will kill everybody once they get started, and my point of view was, two guys had already started tackling him," Norman said. "Maybe they needed some help. ... I said to myself, maybe I have a chance if I get up and I help as well."
When the group decided to take down the gunman, Stone rushed him first, getting slashed several times with a box cutter in the process.
"If anybody would have gotten shot, it would have been Spencer for sure, and we're very lucky that nobody got killed, especially Spencer," Skarlatos said.
He tackled him and seized his weapons, including a box cutter, he said.
"It feels pretty crazy. I never thought I'd be here," Stone said.
Stone suffered wounds in the head and neck, and almost had his thumb cut off.
Despite his injuries, he helped a man who had his throat cut and applied pressure to the neck to prevent him from bleeding out, according to his friends.
Stone, who serves in the U.S. Air Force, was hospitalized for a day. He was released Saturday.
UNKNOWN FRENCH NATIONAL
In addition to the four men, a French national is credited with helping avert a massacre. Authorities did not identify him.
A "French (male) passenger tried courageously to overpower him before the suspect fired several shots," French Interior Minister Bernard Cazeneuve said.
"Then two American passengers intervened and managed to overpower the shooter, immobilized him on the ground and put aside his weaponry."
Applause from world leaders
U.S. President Barack Obama described the three Americans as heroes, and commended and congratulated them in a phone call after the incident.
"The President expressed his profound gratitude for the courage and quick thinking of several passengers, including U.S. service members, who selflessly subdued the attacker," the White House said in a statement.
"It is clear that their heroic actions may have prevented a far worse tragedy."
His French counterpart, Francois Hollande, spoke to them by phone and saluted their bravery. He will present them Monday with Legion of Honor medals, France's highest recognition.
"We often use the word hero, and in this case it has never been more appropriate," U. S. ambassador to France, Jane Hartley, said Sunday.
The suspected gunman, Ayoub El Khazzani, is a Moroccan national, a senior European official said.
He is in custody and undergoing interrogations.
His lawyer told CNN affiliate BFMTV that her client denies he was trying to launch a terror attack. Instead, he boarded the train with the intention of robbing passengers, said Sophie David.