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.cnbc.com