The problem, as you are well aware, is that there is a prominent and powerful Sunni population in Iraq that was the foundation of the support to Saddam Hussein's dictatorship.  Hussein's military also comprised of many Sunni soldiers who were removed from their positions resulting in many of them aligning themselves with al qaeda and ISIL.  The relationship between the American government and the Shiite power bases in Iraq has been conflictual, contentious, and occasionally bloody, but a relationship that has been maintained throughout the post Hussein era.  And, to be clear,  the Americans helped to train the Iraqi forces after their near collapse in fall 2014, as well take par, significantly in supporting the Iraqi military is retaking Mosul- a battle in scope to what took place in WW2.  Support, I might add, that came from the doctrine of the Trump administration- it's first foreign affairs success, and the supporting cast of the previous Obama administration.