The US military knew the strength of the Afghan army as well as the determination of the Taliban. The Taliban have been in wait for
years knowing these days would come. They were all set to fill the void once the US/UN departed. Everyone knew this - there
is and was no secret. The inevitability of this was known years ago.
The thing about it is, the Afghan's knew this too.
Blinken and Co. got the time-line wrong - really no explanation needed - they just have very short term memory and short-sighted
view of an outcome. No debacle, just didn't go the way they wanted it to.