This is a well-preparered and well-executed military operation by the Taliban. First, they negotiated a truce in exchange for a deadline on troop removal. Then they prepared a mobile military force, aimed at conquering big cities quickly (not even the Taliban can really control Afghanistan's countryside). They knew that this domino effect would kill the much larger ANA and the current administration. The media did exactly what they were supposed to do, btw. - this was a media campaign as much as a military one.

This gives us some interesting observations:

Someone must have aided the Taliban buildup. Logistics, intel, control and communication for such a wide-spread campaign ain't trivial. Taliban have no industrial base.

US and, to a certain extend, Afghan officials must have known what was coming. They must have seen the buildup, intercepted communication, etc.

The ANA was unable or unwilling to fight. The obvious counter to this offensive would have been to bolster the defense of a couple of important locations, let them be besieged and smash the local Taliban detachment with superior forces. Even four weeks ago such a strategy could have produced a stalemate. The Taliban likely cannot afford a long-lasting campaign. It looks like the ANA was completely corrupt and was out of the fight before it even began.

What really troubles me is that the Afghan people want the Taliban back. No one seems to protest, let alone resist. There are no volunteers rushing to defend Kabul from the conquerors. No women in arms defending their way of life. It makes me think that the Afghan society as a whole demands and deserves the Taliban rule.

Finally, I wonder why neither China or Russia seem very concerned. Yes, of course they will be happy when US helicopters evacuate burning embassies, but both have their own issues with radical islamists. So maybe they think they can use the Taliban to counter the IS?