The major reason for the US defeat in Afghanistan is that the commanders confused technology and firepower with strategy and were constantly outmanoeuvred by Taliban. Each allied commander in ISAF has his own rules of engagement as a result of which 10 different wars were fought.

Around 20 Generals have commanded ISAF. May it be @StanMcChrystal or Allen each had his own team which confused allies and gave solace to Taliban who took advantage of the fresh leaning curve each time the command changed hands.
Each had his 100- Day plan which never succeeded.