Illustration by Edel Rodriguez for FP

The badly lit, helter-skelter video of Saddam Hussein’s execution said it all: “Moqtada! Moqtada! Moqtada!” a voice cried as the hangmen tightened the noose around Saddam’s neck. Seconds later, the deposed dictator dropped through the floor.

Four years into the American occupation of Iraq, tens of thousands of people are dead and a nation is imploding. And Moqtada al-Sadr, the young, rabble-rousing cleric few people had even heard of when the invasion began, can now plausibly claim to be the most powerful man in the country. Sadr’s power covers the whole spectrum of political possibility: He commands as many allies in the Iraqi Parliament as any single party; and his armed followers permeate Iraq’s security forces, control the streets throughout eastern Baghdad and the Shiite south, and fill the ranks of many of the death squads that terrorize the country’s Sunni minority. The Americans would like to see Moqtada off the scene; many moderate Shiite leaders would like to see him dead. Yet Sadr, still in his 30s, appears unassailable. Indeed, he seems the person most likely to benefit should Iraq sink further into chaos.

Sadr’s rise was less a determined climb than a bubbling up. He ascended on the hopes of his supporters, the millions of downtrodden Shiites who had once looked to his father, Ayatollah Muhammad Sadiq al-Sadr, a scholar and cleric who, along with two other sons, was murdered by Saddam’s gunmen in 1999. The surviving Sadr does not have his...