It may surprise you or not surprise you that when we went to Iraq in 2007, General Petraeus and I said, "Do you know why these people fighting? Show us the data," no study like the Vietcong Motivation & Morale Study (1966), pdf had ever been done. That was the period when the Secretary of Defense and military commander on the ground used to describe the enemy as "evil-doers" and say that "the evil-doers hate us because they hate who we are, not because of what we do." And we said, well, let's look into that.
Based on [a survey of 24,000 people in detention]— which was partly quantitative and partly based on interviews— we determined that 70% of the people were fighting us primarily for economic reasons. Another 20% were fighting us because they had belonged to a former regime group for a formerly dominant social group that had been dispossessed by what happened in Iraq after 2003. They were fighting us to reestablish their position of social or political ascendancy. Less than 10% of the people that we were fighting were motivated predominately by religious motivation or ideology associated with Al-Qaeda. And in fact, a high proportion those 10% were foreigners; they weren't even from Iraq. So that's the sort of data set for Iraq and we did similar surveys in Afghanistan later the tended to show that basically what you have on on the ground in most cases is a very, very small number of irreconcilable fanatics who draw their power from their ability to intimidate, manipulate, and mobilize a much larger group of people who are fighting us primarily because we're in their space and feel like they need to defend themselves rather than because they naturally support that small extremist clique.
— David Kilcullen on Accidental Guerrilla Warfare in the Midst of War (youtube)