Digby wonders if the U.S. didn't actually 
start the sectarian violence?  Not just by invading the country, removing it's leader and creating a vaccum.  But by actually starting a program of violence against Sunni's.
There seems to be 
plenty of evidence that the Pentagon engaged in a program to train Kurd and Shiite militias on special ops/assassination techniques to be used against Sunni "insurgents".   Perhaps Bush had a secret plan to end the war by removing Sunni leaders.
I'm shocked, SHOCKED that such a program wouldn't have actually worked.  And, unfortunately, I wouldn't put it past them having done so.
Update:  
Needlenose reminds us that this is an old story and that later reporting suggests that the Shiites have taken the program very seriously, perfecting it.