A look at Shiite cleric Muqtada al-Sadr and his Mahdi Army militia.

MUQTADA AL-SADR: He is the son of Grand Ayatollah Mohammed Sadiq al-Sadr, a key cleric under Saddam Hussein who was killed in 1999. His followers blame Saddam for his assassination.

The younger al-Sadr launched two major uprisings against U.S. and coalition forces in 2004. He maintained his anti-American stance, but later agreed to work with Iraq's Washington-backed governments. Al-Sadr disappeared from public view more than a year ago at the start of the U.S.-led security crackdown in Baghdad. He is widely believed to be in Iran's holy city of Qom, where he says he is studying to become an ayatollah.

MAHDI ARMY: Formed by al-Sadr in July 2003, the militia has grown into one of the most powerful armed groups in Iraq by offering both protection to Shiites and providing needed community outreach such as clinics and welfare services. The Mahdi Army _ referred to by the U.S. military by its Arabic name Jaish al-Mahdi, or JAM _ began to fragment last year with some factions suspected of forging closer ties with Iran and breaking away from al-Sadr's grip. They factions were apparently brought back into the fold by al-Sadr. Some estimates put the number of hardcore militia as high as 60,000, but some figures are lower. The group takes its name from a messianic figure central to Shiite Islam: the Mahdi, or so-called Hidden Imam, who disappeared as a child in the 9th century. Shiites believe he will return one day to bring justice to Earth. The Mahdi Army, al-Sadr contends, is preparing for the return.

THE SHOWDOWN: Prime Minister Nouri al-Maliki has threatened to ban al-Sadr's followers from provincial elections this fall unless the cleric debands the Mahdi Army. The government began a crackdown last month on what they called breakaway Shiite factions, but it brought a strong backlash from the mainstream Mahdi fighters who have battled Iraqi and U.S. forces in key Shiite centers such as Basra and Sadr City in Baghdad.

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