So it’s like a triangle of violence. The Shias hate the Sunnis. The Sunnis hate the Shias. And both of them hate the Americans. The Americans are there to stop the voilence between the Shia and the Sunni. The sooner they calm down, the sooner the Americans will leave.
So if the Sunnis and the Shia agree to a cease fire till the Americans leave and then wait a little bit after the Americans leave, they can then get back to the business of killing each other without interference by the Americans.