Bing West said this quote

“ By the end of 2004, U.S. operations in Iraq had been rough enough to antagonize the Sunni population without imposing the draconian methods armies habitually employ to control a population.In the spring of 2006, the coalition was losing on the two major fronts that accounted for most of the fighting. In Anbar to the west, al Qaeda controlled the population; in Baghdad to the east, Shiite death squads were driving our the sunnis, while al Qaeda's suicide bombings continued.Yet, the conditions had already been set for a turnaround without precedent in combating an insurgency. In less that three years, two giant institutions steeped in 200 years of traditions-the Army and Marines-adopted new doctrines and turned around a losing war. This was equivalent to GE and Ford starting afresh in new business lines and turning a profit in three years.A lack of soldiers is frequently cited as the basic flaw after the invasion. This is mistaken. There were 140,000 soldiers, plus 100,000 contractors in support roles, in Iraq in 2003. Adding troops would not have accomplished much because the two-headed command...lacked a plan, a counterinsurgency doctrine, and proper training. With the Pentagon's agreement, Bremer had disbanded the Iraqi Army, and the Iraqi police were ineffective. More American troops operating alone under a doctrine of attack and destroy would have exacerbated the rebellion. ”