Iraq: the basic equation

The basic Iraqi equation is pretty simple.

There is a minority of Sunni who have oppressed the majority Shia for a few decades now. They enforced this oppression by killing any Shiite who opposed them.
The Sunni are out of power.
The Americans are in power, but as soon as they leave, the Shia will be in power.
The Shia fully intend to rid Iraq of Sunnis, by starting to kill them while allowing most to move to Syria. That this intent would be realized seems the most likely of several possible outcomes of the US ceding power in Iraq.

It's not clear that this is a good thing, a bad thing, nor a goal worthy of a war.

It does seem clear that both the Sunni and the Shia understand what it takes to end the violence, while the US continues to refuse to accept reality. One or the other side must be in control, this sharing stuff is fiction, and both of them know it.