• After nearly two decades of war, more than 6,000 American lives lost, over 100,000 Afghans killed and more than $2 trillion spent by the U.S., the speed of the Taliban takeover of Afghanistan has shocked the world.
• U.S. defense officials reportedly expected Afghanistan's capital, Kabul, to fall in 90 days. It took less than 10 days.
• Among the key causes, analysts say, are intelligence failures, a more powerful Taliban, corruption, money, cultural differences and willpower.

DUBAI, United Arab Emirates — The world was shocked this week by horrifying scenes of desperate Afghans swarming the tarmac at Kabul's international airport, grasping at their last chance to escape a country now completely overrun by the Taliban.

After nearly two decades of war, more than 6,000 American lives lost, over 100,000 Afghans killed and more than $2 trillion spent by the U.S., the outlook for the country's future was still grim, with regional experts assuming the Taliban would ultimately come to control most of Afghanistan once again.

But few expected a takeover this swift, with so little resistance from the Afghan government and Afghan National Army, the latter of which was funded and trained with $89 billion from the U.S. taxpayer.

"While the end result and bloodletting once we left was never in doubt, the speed of collapse is unreal," one former intelligence official and U.S. Marine who served in Afghanistan told CNBC, requesting anonymity due to professional restrictions.

"Why were the Taliban able to so quickly take over? This is a masterpiece, frankly, operationally," Michael Zacchea, a retired U.S. Marine who led an American-trained Iraqi Army battalion during the Iraq War, told CNBC. "Why were they able to take the country faster than we did in 2001?"

The question has been asked by Americans, Afghans, military veterans and international observers alike — and the answer, much like the Afghanistan conflict itself, is complex, multilayered and tragic.

But among the main causes, analysts say, are intelligence failures, a more powerful Taliban, corruption, money, cultural differences, and simple willpower.

Intelligence failure

The Taliban's rapid takeover of Afghanistan, including its capital and the presidential palace, suggests that U.S. military intelligence failed in its assessment of the situation, according to Bill Roggio, a senior fellow at the Foundation for Defense of Democracies.

"This is an intelligence failure of the highest order," he told CNBC's "Squawk Box Asia" on Monday, adding that it's the "biggest intelligence failure" since the Tet Offensive during the Vietnam War, a campaign of devastating surprise attacks on the U.S. and its allies in 1968.

Roggio said the Taliban pre-positioned equipment and materials, organized, planned and executed a "massive offensive" since early May before beginning its "final assault," while U.S. officials said the local government and military forces should be able to hold out for six months to a year.

Last week, Reuters reported that a U.S. defense official saw Afghanistan's capital, Kabul, falling in 90 days. Instead, that happened on Sunday, less than 10 days after the first provincial capital of Zaranj was taken by the Taliban.

'A collapse in the will to fight'

What's key to note is that the Taliban did not have to fight their way into Afghanistan's provincial capitals but rather brokered a series of surrenders, says Jack Watling, a research fellow for land warfare and military sciences at the Royal United Services Institute in London. Over the last few years of fighting, the group managed to gain control of some 50% of the country by seizing rural areas.

And when they began making headway in cities, many Afghan forces gave in to them, convinced that the government in Kabul would not back them up.

"The Taliban would infiltrate urban areas, assassinating key people like pilots, threatening the families of commanders, saying if you capitulate, you'll save your family," Watling said.

"A lot of people, because they lacked confidence that Kabul would be able to save them, capitulated." More and more people chose this route, "so there was very little fighting, which is why it suddenly happened so fast," he added. 

"The speed is not a reflection of military capability, it is a reflection of a collapse in will to fight."

The news from the Biden administration of the full U.S. withdrawal sped this up, said Stephen Biddle, professor of international and public affairs at Columbia University.

"When the U.S. announced a total withdrawal, that sent a signal to Afghan soldiers and police that the end was near, and converted chronically poor motivation into acute collapse as nobody wanted to be the last man standing after the others gave up," he explained.

"Once the signal was sent, contagion dynamics thus took over and the collapse snowballed with increasing speed and virtually no actual fighting," Biddle added.

In April, Biden ordered the Pentagon to withdraw U.S. troops from Afghanistan by Sept. 11, a decision he said was made in lockstep with NATO coalition forces. On Monday, the president defended his decision to leave the country and placed the blame squarely on the Afghan national government.

"American troops cannot and should not be fighting in a war and dying in a war that Afghan forces are not willing to fight for themselves," Biden said. "We gave them every chance to determine their own future. We could not provide them with the will to fight for that future," he added.

Afghan President Ashraf Ghani himself fled the country on Sunday evening as the Taliban entered the presidential palace and declared the war "over." Ghani said he fled to prevent "a flood of bloodshed."

"The Taliban have won with the judgment of their swords and guns, and are now responsible for the honor, property and self-preservation of their countrymen," Ghani said.

Despite being vastly outnumbered by the Afghan military, which has long been assisted by U.S. and NATO coalition forces, the Taliban carried out a succession of shocking battlefield gains in recent weeks.

On Sunday, the Taliban arrived at their last destination and seized the presidential palace in Kabul.

"The swift Taliban takeover shows how utterly dependent the Afghan state was on the U.S.-led coalition, materially and psychologically. Even before the U.S. withdrawal, the Afghan government and security forces were fraying at the seams," said John Ciorciari, director of the International Policy and Weiser Diplomacy Center at the University of Michigan's School of Public Policy.

"Soon after the U.S. pullout began, Afghan troops and officials began jumping ship, either to appease the Taliban or to retreat into old ethnic militias. The Taliban takeover will not bring peace. As the dust settles, many U.S.-trained fighters will likely regroup along ethnic lines to fight again," he added.

Taliban 'much more adept' militarily

Not everyone believes the U.S. troop withdrawal is to blame for the chaos in Afghanistan today.

Kirsten Fontenrose, director of the Scowcroft Middle East Security Initiative at the Atlantic Council, said the Taliban has become more effective since the 1990s.

"They've become much more adept ... militarily and non-militarily in terms of pursuing the same objective they have — which is establishing an Islamic emirate in Afghanistan," she told CNBC's "Squawk Box Europe" on Monday.

"The U.S. withdrawal is not the reason the Afghan government was outmaneuvered," she added.

Fontenrose said the Taliban surrounded the capital of Kabul, cut off supply lines that government forces needed, and have also have grown in numbers while developing new strategies.

"They use social media as lethally as they do sniper rifles. They've used coercion to pressure local tribal leaders, they've used pretty simple but effective text message campaigns to threaten local Afghans working with the U.S. and with other foreign efforts," she described.

The Taliban also lets ground commanders make decisions, and brings people into captured territories to provide small-scale social services to the residents.

That has allowed the group to "outmaneuver" Afghan and foreign forces in terms of effectively appealing, co-opting or coercing the local population into supporting — or not opposing — them, she added.

Afghan government corruption and military weakness

Had the Taliban engaged in a full military onslaught and faced resistance, the blitz of the country would have taken longer — but it still would have happened, Watling believes.

"I think the Taliban would have still won," he said. "And this is because the Afghan National Army is comprised of lots of units that are systemically corrupt, have no effective command and control, they don't know how many people are in their own units, most of their equipment has been taken apart, stolen and sold off, and so they were a completely dysfunctional force."

It's also because the Afghan military is woefully underpaid, underfed and undercompensated by the leadership in Kabul.

The "soldiers in many cases have not been fed very well, very rarely been paid and been on duty for a long time away from home ... and were not well led," Watling added, a tactical failure that resulted in heavy casualties to the tune of about 40 soldiers a day for the past several years.

Many army units would sell their equipment to the Taliban for cash, and there were frequent desertions that went unaccounted for, leaving inflated troop numbers on the books.

How little Americans 'understand Afghanistan'

Central to understanding America's failure in Afghanistan also comes down to understanding the country's history and its culture — and how drastically it differs from any Western nation.

"There's never been a central government in Afghanistan. To think we could establish one was a fool's errand," said the former U.S. intelligence officer and Afghan War veteran. "The 'surprise' at the Taliban regaining power shows just how little Americans, from top to bottom, understand Afghanistan."

Afghanistan is a country of numerous tribes, languages, ethnicities and religious sects, and Washington and its NATO allies were attempting to turn it into a unified democracy premised on largely Western values.

"There was a fundamental failure to understand what the Afghans wanted," Zacchea, who trained an Iraqi battalion in 2004, said. "We assumed they wanted what we had — liberal democracy, Judeo-Christian values ... And think they'd just automatically convert. And that is not the case."

Tribal alliances in Afghanistan very often supersede national ones, or loyalties follow money and power. And part of the Taliban's strength lay in the fact that as Pashtuns, they belonged to the largest ethnic group in Afghanistan.

"Meanwhile," the former U.S. intelligence official said, "we basically supported a hodgepodge of ethnic minorities, who never had the capability of unifying the country."

"We did not understand the tribal dynamics, we never did," Zacchea said. "We think everybody wants what we have. It's cultural obtuseness, obliviousness to their reality and their lived experience." 

The nature of the U.S.-brokered cease-fire with the Taliban in early 2020 also further weakened the Afghan government's image: Negotiations led by the Trump administration left out the elected leadership in Kabul, which "destroyed the Afghan government's legitimacy" at a time when it already had little respect from local communities, said Watling.

Afghans across the country of 39 million have expressed acute fear for their country's future — especially women, who following the U.S. invasion in 2001 were able to go to school for the first time since the Taliban first took control of Afghanistan in 1996. For many Afghan War veterans, bringing some of these basic freedoms to Afghans made their sacrifices worth it.

Now, those achievements are set to vanish, lamented one American veteran who served as an infantryman in the country in 2011.

"I have no regrets about what I did in Afghanistan," the former Marine told CNBC, requesting his name be withheld due to job restrictions on speaking to the press.

"I just feel devastated for the people I saw over my time there when they were kids. Now they're teens, and I can only imagine what they are going through."

— Amanda Macias contributed to this report from Washington, Natasha Turak contributed from Dubai, and Abigail Ng contributed from Singapore.