We are looking a massive disaster in the face in Iraq as each of these things come to pass...

Shiite death squads continue to roam unabated.
Sunnis continue to respond to attacks with bombings.
Al Sadr orders his group to pull out of the Maliki Govt.
Sunnis pull out of the Maliki Govt because the Shiite death squads are not reigned in.
The Maliki Govt falls with no other Iraqi groups left to step in and restore order.
US troops are placed in the position of having no recognized legitimate Iraqi
Govt to side with in trying to quell sectarian fighting.
When the Maliki Govt falls, Shiite and Sunni groups outside of Iraq pour into Iraq money and arms
Oil fields come under attack, and violence spreads outside Iraq
US troops and ships come under direct attack as they try to secure flow of oil
US troops have to begin deploying out of Iraq under fire from all sides

These events may not happen in any particular order, and all may not occur. But the case can certainly be made for each of them coming to pass, and the consequences will be devastating.

Leave Iraq.........which would be the civil thing to do
in a Civil war.

Saudi Arabia getting involved--they're not going to stand around while Sunni's get slaughtered by Shiites.
Also, Iran will get more involved especially if S.A. jumps in.