Sadr, as well as being a high-profile religious figure, leads the Sadrist political party, which primarily represents the one million residents of Sadr City, an impoverished Baghdad suburb. He also leads the Jaysh al-Mahdi. Also known as the Mahdi Militia, it was responsible for years of brutal attacks against Iraq's Sunni Arab minority. The Mahdi Militia began as part of the Shiite uprising in 2004 to provide security for Sadr City, but later turned to a campaign of violence against Sunni neighborhoods and mosques. In July 2006, masked Shiite militiamen, some of whom were likely associated with the Mahdi Militia, slaughtered 50 civilians in a Sunni neighborhood of Baghdad. The attack began a sectarian conflict that would claim 3,000 Iraqi lives that month alone.
The militia's area of control and anti-Sunni violence continued until March 2008, when Prime Minister Maliki and U.S. military commanders joined in a campaign against the Mahdi Militia. They succeeded not only in rolling back much of the Mahdi Militia's control but in forcing Sadr to recognize the authority of Maliki's government, backed by the U.S. Since then, Sadr has shifted his energy from violence to politics, emerging as a "kingmaker" in the split Parliament and hosting a non-binding vote to demonstrate his influence. While it's difficult to watch a violent anti-American win so much power, Sadr's political engagement has so far been a good thing for Iraqi and American interests. Rather than expressing their desire for U.S. troops to leave by bombing them, for example, the Sadrists can pass legislation or make diplomatic entreaties paving the way for American departure.
Sadr faces a choice between, as Malcolm X put it, the ballot or the bullet. The great looming danger for Iraq's nascent democracy is that Parliament's fragile political coalition falls apart. If the Sadrists feel compelled to abandon their political engagement for a return to violence, wider political disintegration is a possibility. While they've given no sign of doing that, increased attacks from the radical Sunni al-Qaeda terrorists risk spurring the Sadrists to retaliate. If Sadr can resist the temptation to once again unleash his militia on Sunni Arabs, then today's attack will be unlikely to bring a return to sectarian violence. But if the Shiite cleric feels compelled to retaliate against Sunni neighborhoods, as he did in response to such attacks during Iraq's worst years, it could mean precipitous political destabilization or even spiraling sectarian violence, handing al-Qaeda terrorists exactly what they want.
Update: Sadr has ordered some of his Mahdi militia to reactivate. While for now he has only told them to guard certain mosques, their presence on Baghdad's streets risks escalating an already tense situation.
Image: Moktada al-Sadr speaking in Basra in 2006. Khaldoon Zubeir/Getty Images