- CNN's Ivan Watson was in Kabul when the city was seized from the Taliban
- US involvement uncertain as Donald Trump prepares to take presidency
(CNN)It was a moment I will never forget.
Fifteen years ago this week, I was marching into the capital of Afghanistan along with thousands of cheering, triumphant Afghan fighters from the US backed-Northern Alliance.
Suddenly, something struck me in my face. It was a silver foil-wrapped candy thrown by the cheering residents lining the streets as they welcomed the arriving fighters.
The hard, celebratory chocolate-in-the-face was almost as abrupt and startling as the sudden collapse of the Taliban.
Just hours earlier, the militants had abandoned their front line positions and fled Kabul en masse after enduring barely six weeks of US airstrikes.
That swift military victory took place on November 13, 2001.
At the time, I never imagined that Afghanistan would become the longest foreign war in US history.
For Afghans like my good friend Najib Sharifi, who was only 19 years old at the time, the defeat of the men who had imposed their strict version of Islam on Kabul was a pivotal moment.
"It was probably the happiest day of my life," he said.
Celebration and retaliation after the fall
Najib recalled how 15 years ago residents forced one of the Taliban's most notorious religious police enforcers to dance for hours in a public square to music that had been banned by the former regime.
"If he stopped dancing people would go and beat him up," Najib said, laughing out loud at the memory. Najib is now the director of the Afghan Journalists' Safety Committee.
Some Afghans were more vindictive. That day, I stared at the corpse of a slain man -- allegedly a Taliban militant from Pakistan -- who was left beaten to death under a rusty basketball hoop in Kabul's leafy Share Naw Park.
The hunt for Osama Bin Laden
For the US, the overthrow of the Taliban was a military response to the trauma of the September 11 terror attacks, which had been planned and orchestrated by Al Qaeda leader Osama Bin Laden.
In the aftermath, the Taliban refused to hand over their ally, Bin Laden.
It took the US another 10 years before American Special Forces soldiers killed the Al Qaeda leader during a night-time raid in the Pakistani city of Abbottabad.
In the interim, Washington embarked on a costly campaign of nation-building in a country devastated by decades of invasion and civil war.
Then-President George W. Bush invoked memories of a World War II-era Marshall Plan for Afghanistan, after pledging billions of dollars to help rebuild the country.
There have been bright spots since. Several times, Afghans have lined up in their finest clothes to vote in national elections.
The school doors opened to millions of girls who had been denied education by the Taliban.
Companies built a national cell phone grid, bringing an end to the days when Afghans had to travel to neighboring Pakistan just to make a phone call.
But the establishment of elected, Western-backed governments in Kabul was marred by allegations of rampant corruption.
Even though the US war in Afghanistan cost some $780 billion US taxpayer dollars -- according to a study by Brown University -- there are still dirt roads and chronic power outages in the center of Kabul.
Today, a US-brokered power-sharing agreement between two political factions in the Afghan government teeters on the brink of collapse.
The Taliban's resurgence
As for the Taliban, it has regrouped and begun fighting back. The conflict cost at least 2,380 American lives and killed tens of thousands of Afghan civilians.
According to recent US military estimates, the Taliban either controls or is currently battling to capture territory that's home to more than a third of the country's population.
The days when a foreign journalist could travel in relative safety for hours on bone-breaking dirt roads visiting remote corners of the country are long gone.
Today, due to the threat of kidnapping and terror attacks, US diplomats don't even drive the short distance from Kabul Airport to the US Embassy. Instead, they fly by helicopter.
"I could never imagine that the Taliban would be back at the gates of Kabul," said Najib, shaking his head in dismay.
"How could one imagine that a disorganized group like the Taliban would fight with all the world power and gain so much ground?" he asked.
US troops still there
There are still around 9,800 US troops deployed in Afghanistan, along with several thousand soldiers from more than 30 other countries. The Afghan security forces now do the bulk of the fighting, but this year they are also suffering record high casualties.
A Western official in Kabul sadly predicted that depending on how things ago, Westerners could be fleeing from helicopters off the roof of diplomatic buildings within two years, in scenes frighteningly reminiscent of the US evacuation from Saigon in 1975.
Some observers argue it's time to bring an end to the conflict.
"It's our longest war and we don't seem to be making any progress," said retired US Air Force lieutenant colonel Rick Francona.
"It brings back for those of us who served in Vietnam, bad memories of that period in time," he added.
But an abrupt US withdrawal under a future Trump Administration could imperil an entire generation of Afghans raised on promises from the international community.
"The only option out of this war is to defeat those extremist, medieval, dark-minded terrorists," said Sharifi.
Several months after the 2001 fall of Kabul, I remember being stunned at the sight of US soldiers and Apache attack helicopters deployed in large numbers in dusty mountains of Shah-i-Kot in Eastern Afghanistan.
Fifteen years ago, the world was not accustomed to US military forces fighting wars without end in the Middle East and Central Asia.
But the swift collapse of the Taliban ushered in a new era of US military adventurism and nation-building. It remains to be seen what President-Elect Trump will do with that troubled legacy.