Wednesday marks a new era for the war in Afghanistan and the young people who make up the bulk of enlistees. It is the first day someone born after the terrorist attacks can enlist, at age 17, and begin a path to serve in the seemingly endless war launched in response to those attacks.

That is a mind-bending prospect; troops were once partially motivated to enlist because of the attacks. Now, 17 years later, the unfinished war grows further from the events that created it.

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The dividing line between troops who enlisted before and after 9/11 was initially stark, veterans have said.

Brandon Friedman was commissioned in the peacetime Army of 2000 and took over an infantry platoon five days after the attacks.

He later led the platoon in Afghanistan in 2002. Those men had all enlisted before the attacks, he said, and had joined for a number of reasons — to test their mettle, earn college benefits or maybe to escape dim prospects at home.

But the replacements he received by 2003, who had all enlisted in the wake of 9/11, said they joined for different reasons.

“It was a galvanizing time,” he told The Washington Post on Tuesday. “We sort of romanticize it now, but there was a lot of unity and sense of purpose in the country and in the military.”

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About 5.5 million troops have served since 9/11, and nearly 7,000 have been killed in Iraq and Afghanistan combined.

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Kayla Williams, a former Army linguist, was in Arabic class during the attacks. Like Friedman, she later met recruits newly inspired to fight. In recent years, however, enlistees are less likely to say 9/11 played a role in their decision to join the military, said Williams, now director of the military, veterans and society program at the Center for a New American Security, a Washington think tank.

Pentagon data shows an 8 percent surge in the propensity for young men to enlist right after the attacks, continuing through 2005.

Now, recruits report motivations that mirror those of their pre-9/11 forebears; they join to pursue adventure, secure benefits or are drawn to aspects of honor, she told The Post.

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The propensity to enlist has been in steady decline, said Phil Carter, a military and veterans researcher at Rand Corp. Researchers are not sure why.

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It could be a strong economy, the perceptions of military service as physically and mentally damaging, the wars themselves, or all the above, Carter said.

There may have been spikes of interest in military service after the attacks, said Carter, a former Army officer. But he said 9/11 has always been a modest reason people have enlisted, never a major one, such as the economy — though sometimes 9/11 is an easier way to explain the often complicated reasons someone might enlist.

The length of the wars in Afghanistan and Iraq, however, has produced a wide spectrum of why and when people enlist, and the connection to 9/11 has all but faded.

Jon Gillis was in fifth grade on 9/11, and his friends had parents in the Pentagon during the attacks. But that was not a specific driver, he said.

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Gillis enlisted as a college graduate in 2013 after becoming close with a Marine veteran at Georgetown University. By then, the wars had passed their inflection points of media attention and combat deaths.

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He entered a Corps in transition, where Marines hoped for combat rotations and were dismayed by dwindling chances to fight in Afghanistan, as their leaders had. Gillis deployed to Romania instead.

He left active duty last year, as fresh-faced, 18-year-old Marines arrived at his unit. Barracks talk focused on the generational gaps, he said, with 9/11 as the reference point. “They know it happened, but there is no memory attached to it,” Gillis said.

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The war in Afghanistan is arguably more of a legacy for the post-9/11 military than the terrorist attacks themselves. There are far fewer troops there than in past years, but insurgents still claim lives.

The Taliban has focused on insider attacks to kill U.S. troops on their own bases, and on driving a wedge into the perpetually struggling mission to train Afghan forces. That mission is entering its second decade.

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Command Sgt. Maj. Timothy A. Bolyard, 42, was the most senior enlisted man in the Army's unit created to train and advise front-line Afghan forces. He was killed by an Afghan policeman on Sept. 3, on his seventh deployment.

Bolyard could recall a world before 9/11. But Cpl. Joseph Maciel, a 20-year-old soldier in the same adviser unit, could not. He was killed in an insider attack on July 7.

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Maciel would have been about 3 years old in 2001, learning colors and shapes when combat in Afghanistan was set in motion.

Now the war is poised to grow older than he did.