Most of the civilian and military casualties (95%+) were of the Sunni people groups and the al-Qaeda folk (al-qaeda were mostly foreigners).

During the last years of the US and allies presence in Iraq (2010-2011) the Shia groups were conducting more attacks since the Sunni resistance went into infighting (non-Salafi elements vs Salafi elements, the latter dominated with foreigners). The Salafi Khavarij were causing a lot of problems for the local Sunni population: killing them, raping them etc....

Robert Gate's point about the 'residual' troops were correct. Today in 2018, about 3500 American troops are present. But not in active positions. And they are all based in Iraq's Kurdistan Autonomous region. Insh'Allah they will be kicked out as well - hopefully without bloodshed.