Why Did The Afghan Army Fall?
To understand how the Afghan army fell, we think of it from the perspective of an Afghan soldier.
There were a lot of very brave Afghan troops. Some were excellent units. The vast majority, if presented with the choice of fighting the Taliban to hold the country or to merely surrender and turn the country over, would choose to fight. But, that is not the choice they were presented with.
The US essentially left and took their maintenance teams that fixed Afghan aircraft and other equipment. Additionally, there was no more air support.
The Taliban took advantage and started going to isolated units, offering amnesty and a safe passage to Kabul airport if they surrendered, or else they'd all be killed. Concentrate force on isolated units to where an outcome is obvious.
Troops started to surrender a few months back.
This further hurts morale and indicates to other Afghan forces that their comrades may leave them to surrender, and surrender is a viable option to save their life.
Rinse and repeat.
The option is not so much fight vs. surrender as a group.
The group's choice appeared obvious, so the choice individual servicemen faced was to either die for sure, or walk away and hope I can get to my family and protect them.
For a few billion dollars a year we could have kept maintenance teams on the ground and some special forces and advisors.. a couple thousand folks... and helped instill backbone for the Afghans and keep morale up and prevent this from happening.
Other takes on potential happenings from the fallout
1. Ton of rare earth metals in Afghanistan. You can bet China will be going for these.
2. Look at the Indian media coverage.
The Indian media knows the Taliban are effectively an extension of Pakistan (If not for that, we would have beat them a long time ago. They had support and a safe haven the whole time). The Indians do not want an Islamist government aligned with Pakistan politically and China commercially on their doorstep.
They have a strong interest in supporting whatever anti-Taliban resistance emerges (back to Panjshir here).
At the end of the day, between the ineptitude and cowardice of the Ghani administration (had Saleh been President, this would likely have been different), there wasn't much of a choice.
People forget the number of casualties the Afghan soldiers were taking on a regular basis and were still showing up for work the next day.