Mr. Chair, I've been thinking about this question for the last 15 years, since serving there in 2007. It could be a number of different areas, but I'm not sure the premise of “where did we fail” is accurate because we provided some excellent training and some excellent equipment.
I'll recommend an interesting book. It's called A Question of Command by Mark Moyar. It discusses the importance of leadership in counterinsurgency and insurgency operations and how the elites of a country have to take ownership of the fight. It talks about how the elites have to lead and set the example for all to follow. With the endemic corruption in the country at the time, it was hard for the elites to have that credibility with the average person.
Again, it's just my own personal opinion, but one I've reflected on a lot.