A UCLA study assesses the success of the surge by using satellites to study light patterns in Iraq:
"Essentially, our interpretation is that violence has declined in Baghdad because of intercommunal violence that reached a climax as the surge was beginning," said lead author John Agnew, a UCLA professor of geography and authority on ethnic conflict. "By the launch of the surge, many of the targets of conflict had either been killed or fled the country, and they turned off the lights when they left."
Petraeus was pushing at an open door - not that what he did wasn't still remarkable. And so the decline in violence cannot be ascribed entirely to the surge at all:
"If the surge had truly 'worked,' we would expect to see a steady increase in night-light output over time, as electrical infrastructure continued to be repaired and restored, with little discrimination across neighborhoods," said co-author Thomas Gillespie, an associate professor of geography at UCLA. "Instead, we found that the night-light signature diminished in only in certain neighborhoods, and the pattern appears to be associated with ethno-sectarian violence and neighborhood ethnic cleansing."
(Hat tip: Mark Thoma)