Three main motivations present themselves: control of petroleum smuggling, staying in power (including keeping U.S. troops around to ensure it), and the achievement of a Shiite super-province in the south. A southern super-province would spell a soft partition of the country, benefiting Shiites in the long term while cutting Sunnis out of substantial oil revenues, both licit and illicit. But all of the motivations have to do with something President Bush established as a benchmark in January 2007: upcoming provincial elections.
The Sadr Movement leaders themselves are convinced that the recent setting of a date for provincial elections, on Oct. 1, 2008, and al-Maliki's desire to improve the government's position in advance of the elections, precipitated the prime minister's attack. It is widely thought that the Sadrists might sweep to power in the provinces in free and fair elections, since the electorate is deeply dissatisfied with the performance of the major incumbent party in the southern provinces, the Islamic Supreme Council of Abdul Aziz al-Hakim.
It seems an open question how much al-Hakim's influence led Maliki to undertake the attack.Or perhaps he had other influences, such as the Americans.
Al-Hakim and his party also favor an autonomous Shi'ite province in southern Iraq, much like the Kurds already have in the north. This is fiercely opposed by the nationalistic Sadrists.
And then there is the super-province. On the same day that Bergner spoke, a Sadrist leader told the Times of Baghdad, "The objective of the operations in Basra is to impose a provincial confederacy on the south, which the Sadr Movement opposes." The reference to a provincial confederacy, confusingly called "federalism" in Iraq, is to the plan of ISCI leader Abdul Aziz al-Hakim to establish an eight-province regional government for the Shiite south. Were the Sadrists to win the southern provinces in October's provincial elections, they would halt any move toward such a confederacy, since they favor strong central government on the French model and view al-Hakim's plan for a Shiite super-province as the first stage in a soft partition of Iraq.
This doesn't sound like a very good idea, and ultimately, I hope Iraq isn't partitioned (with the exception of the Kurdish area, which is likely lost to the country). We should keep in mind though that solutions similar to al-Hakim's plan are favored by plenty of US liberal policy, such as Joe Biden.
The most important difference between the government and the Sadrists is of course their stands on US troops in Iraq.
Indeed, the Sadr Movement, which helped bring al-Maliki to power in spring of 2006, broke with him precisely over his refusal to demand that the U.S. set a timetable for troop withdrawal from Iraq. The Sadrists also objected to al-Maliki's direct meetings with Bush, which they saw as a humiliating capitulation to colonialism. In summer of 2007, the Sadrists withdrew their ministers from al-Maliki's cabinet.
I also recommend Matt Yglesias's article on the Atlantic's website.
Years ago, ISCI and Dawa decided to collaborate with the U.S. occupation authorities whereas the Sadr movement deemed it illegitimate and demanded withdrawal. That decision locked us in to the current path and reminds that to a considerable extent the goal of the American military presence in Iraq is simply to continue the American military presence in Iraq and that means forging alliances with whichever Iraqi groups are willing to have us.