Iraq’s most powerful Shiite militia chief ordered his gunmen today to halt all hostilities for six months to restore his Mahdi Army to credibility in the eyes of Iraqis shaken by a rash of Shiite-on-Shiite violence.

The stunning move, coupled with a vow by the anti-American cleric Muqtada Sadr to cease attacks on U.S. forces in Iraq, may also have been aimed at elevating his standing with Iraqis and their neighbors by demonstrating that he has the power to make peace or to destroy it.

Sadr’s announcement through his offices in Najaf followed a deadly clash in the holy city of Karbala earlier this week that killed at least 52 people and injured 300 in fighting blamed on his Mahdi militiamen and their rivals within the Badr Brigade, the armed wing of the country’s biggest Shiite political force, the Supreme Islamic Iraqi Council.