Juan Cole tells us what a successful surge means:

A crucial element in the fall of violence from the catastrophic levels of summer,2006, was the ethnic cleansing of Baghdad of its Sunnis. I wrote in mid-July:

"As best I can piece it together, what actually seems to have happened was that the escalation troops began by disarming the Sunni Arabs in Baghdad. Once these Sunnis were left helpless, the Shiite militias came in at night and ethnically cleansed them. Shaab district near Adhamiya had been a mixed neighborhood. It ended up with almost no Sunnis. Baghdad in the course of 2007 went from 65% Shiite to at least 75% Shiite and maybe more. My thesis would be that the US inadvertently allowed the chasing of hundreds of thousands of Sunni Arabs out of Baghdad (and many of them had to go all the way to Syria for refuge).