The Taliban walked it in August 2021. How could this have happened?

It will remain one of the most humiliating defeats for the US and other coalition military in the history of warfare. After spending $1 trillion to turn Afghanistan into a reasonably functioning democracy and to keep the Taliban away from regaining power in Kabul, and building an army and police force of 330,000 personnel to keep the elected government and the people of Afghanistan safe, everything was just thrown away. No one, and I mean no one, in the Washington establishment, including the most senior military at the Pentagon and in Afghanistan itself imagined that the Taliban would seize back control in such a fantastically quick time. The worst scenario was perhaps a year. But it was just weeks. Why? Because as soon as the Doha agreement had been signed, the Taliban knew that the US had lost interest and they probably gauged that the 330,000 armed Afghans trained by the US and Britain and all the other members of the coalition, would just turn tail and wouldn't put up a fight. How right they were. All that money wasted on an army that wouldn't be prepared to fight for their country when the chips were down. And of course the Kabul government was corrupt and useless and thought only of themselves. The president, Ashraf Ghani, got out of the country as quickly as he could. It was in every way a massive and disgraceful shambles followed by the even more disastrous withdrawal by all the US-led troops. The Pentagon chiefs wanted 4,500 US troops left behind and then agreed to limit it to 2,500 but no lower. But Joe Biden said no. So that was it. Not a single US boot left on the ground and the Taliban back in charge to do their worst. Yes, definitely, even more than Vietnam, the worst defeat in history.