[O]fficial Washington has never understood the real reason for which rates of civilian deaths fell dramatically in Iraq in late 2007 and through 2008, compared to the almost apocalyptic death rate in 2006-2007 during the Sunni-Shiite civil war kicked off by the Feb. 2006 bombing of the Askariya "Golden Domed" shrine in Samarra.
Beginning in a big way in summer of 2006 and continuing for at least a year, the Shiites of Baghdad and its environs determinedly and systematically ethnically cleansed the Sunnis from the capital. I figure that over a million people were likely displaced. Mixed neighborhoods such as Shaab became wholly Shiite. Baghdad went from being 50/50 Sunni-Shiite, more or less, in 2003 to being perhaps 85%-90% Shiite today. Much of the violence of the civil war period was the result of neighborhood fighting between adherents of the two branches of Islam, so when the Sunnis were expelled (many of them all the way to Amman and Damascus), the violence naturally declined substantially.
Rubin thinks that the violence declined because the US government began being willing to enlist Sunni militiamen to fight radical fundamentalists and Baathists. But the Sunnis took the deal in part because they were losing so badly. And, the main effect of the Awakening Councils or Sons of Iraq was in al-Anbar Province, which only has a little over a million people out of Iraq's 27 million, not in Baghdad. In the capital they probably just stopped the ethnic cleansing of Sunnis.
The reasons the Shiites won the civil war in Iraq include:
1) Shiites were the majority, with 60% of the population;
2) Shiites had militias such as the Badr Corps and the Mahdi Army to carry out the ethnic cleansing;
3) Shiites had gained control of an oil state and had significant monetary resources;
4) Next-door Shiite Iran offered enormous resources and facilities to the Iraqi Shiites, helping them avoid being strangled by the Sunni Arabs of the west and north. In essence, the US caught a big break insofar as its main regional enemy happened to have the same basic objectives in Iraq as did the US, reinforcing Washington's policies.
5) Most Shiites and their Kurdish allies (altogether some 80% of the population) saw the al-Maliki government as legitimate, though most Sunni Arabs did not.
6) Shiites had gained control of the newly trained army and security forces and could deploy them against Sunnis, since the new recruits were largely literate, increasingly well-trained, and motivated to stop Sunni violence against their relatives;
7) US troops disarmed the Sunnis in the capital first, before turning to Shiite militias, leaving the Sunnis helpless before 2) and 3) above; and
8) Most Sunni Arabs in Iraq were and are secular nationalists who resented the religious extremism of many of the guerrillas, and whose tribes began to have a feud with the Islamic State of Iraq because it bombed Sunni young men seeking recruitment into the national police.
Seeking a Homeland, a Progressive Christian blogger from out neck of the woods, also has something to say about the Afghanistan v. Vietnam connection.