I would like someone to clarify just what our mission was

[ProfessorGAC]

16. Revenge

And many, irrespective of political tilt, were volcanically angry about 9/11.
Al Qaida needed to pay. The Taliban harbored them, and "you're either with us or against us". So, they had to pay, too.
But, as heads cooled, the "sales" point became liberating the Afghan people, especially women & girls, because only for right wingers was revenge a good enough reason.
Nation building ideas was marketing.
When bin Laden gave 43 the slip, some insiders started thinking dollar signs.
So, more marketing came into play (democracy, freedom, blah blah) to sell reasons to stay.
Once we got bin Laden in Pakistan, we should have started a "no deadline" withdrawal, taking anybody with us that wanted to go.
But, all in all, Afghanistan was about vengeance. Any other reasons were marketing plans.