An article in
The Washington Post explains the US's role in the Iraq Civil War:
"The plan is to prevent a civil war, and to the extent one were to occur, to have the... Iraqi security forces deal with it to the extent they're able to," Rumsfeld told the Senate Appropriations Committee when pressed to explain how the United States intended to respond should Iraq descend wholesale into internecine strife.
If civil war becomes reality, "it's very clear that the Iraqi forces will handle it, but they'll handle it with our help," Abizaid said later when asked to elaborate on Rumsfeld's remark.
First of all, it is too late to "prevent" a civil war. Either it is already happening, or the runup to it is already past the point of no return.
Secondly, if and when things really heat up, most members of the security forces will likely
join one side or the other. What does the US military do then? Shoot everybody? Or pick a side? Either option would be disasterous for the US.
When you make really bad decisions -- like launching an unprovoked attack against a tribal Middle Eastern Muslim country without expecting any popular resistance or internal violence -- you leave youself with no good options. Probably the best way for the US is to "handle" the Iraq Civil War is to get out of the way.
more thoughts
here