Twenty years ago the Taliban had been in power for 7 years and were a normal, static army. Part of the initial US attack involved taking out a Taliban airfield. There was still resistance to them, through the Northern Alliance, even though Massoud had been killed 4-5 months before, on 9/9, I think. People were highly motivated to get rid of them, having suffered through their crimes (though they were never ISIS-level). They were also a regional army, mostly from the Pashtuns in the southeast and not popular with the other Afghans.
2001's Taliban were not a guerrilla army, they were a static government "army". And one that had grown fat on bullying a population that hated them. They weren't even, really, the guys that had taken out the USSR, just some guys that waltzed in to take out the warlords, 2 years after Najibullah had been ousted. Most of all, they were fixed in cities and barracks.
Today's Taliban have had 20 years of evading US airstrikes as hardened guerrillas. The Afghan army they are facing supposedly hasn't been paid for months and suffers recurring switches of its officers from President Ghani's mismanagement.
Whatever is happening is due to Afghan army demotivation, or, as been claimed as well, lack of support by the government (troops not getting supplies and reinforcement). Not through a supposed let-them-take-over-now by the, extremely frustrated at this point, US military.
- Afghan forces surrender
- Taliban can either melt away as they've learned to do in the last 20 years, take some human shields, or just remain too close to civilians and opposing Army troops to allow easy aerial targeting, something known as "hugging the enemy" from Stalingrad.
- The Taliban know that they won't get countered by US ground forces at this point. Expect them to stay well away from foreign nationals as they get evacuated to avoid giving reasons for US and NATO troops to intervene.
A war like this can't be won without (effective) "boots on the ground".
We need to look
* at the dynamics of why the ANA either can't fight effectively or chooses not to fight effectively, because on paper they have what it takes to defend much more strongly.
We also need to understand why the Taliban are so effective throughout the country. Normally, theywould be expected to operate best in the southeast, next to Pakistan. Taking over Herat and Mazir-i-Sharif, so quickly, is not a good sign at all. Maybe they pre-distributed troops there, against weaker ANA units. Maybe they just have really become more supported by larger segments of the population everywhere.
Most of all, the US needs to take a long hard look at its bad track record of enabling stable governments that are supported by locals in these instances. Possibly because they mostly pick leaders that say what the US wants to hear, rather than what the population wants to hear.
* "We need to look" might very well end up, "historians will need to look", because this fights looks all but over.