Almost all gains against IS in Iraq have been led by Shia militias, and they have assumed command of retaken areas. However, because of the anti-Sunni brutality of the Shia militias many Iraqis see IS as best-of-the-worst answer to instability. This is a disaster for attempts to unify the country.
Something different happened in Ramadi. “The main shock troops were the elite units of the Iraqi Counter-Terrorism Service (ICTS), a US-trained outfit answering directly to the prime minister.” [BBC]
The Ramadi model was IS-effective, but it physically destroyed Ramadi and displaced most of the population who suffered massive losses. Their return, however, is a sign that they don’t feel threatened by the non-Shia liberators. It gives the Sunni a sense of belonging, and encourages their tribal leaders to join the fight against IS. In 2006 the effectiveness of enlisting the tribal fighters in driving out most of Al-Qaeda was proven. It can work again if the Sunni believe that victory will not install Shia as territorial controllers.
Can this be scaled up to take Mosul, a city 10 times larger than Ramadi? This will require commitments by the U.S. and Iraq’s central government to increase the recruitment and training of the ICTS, and a consistent resistance to using Shia militias in major operations. It seems the only sensible way to move forward.