OBVIOUSLY, LIFE IN BAGHDAD wasn't just clowns and booze. In early February, we left the Green Zone for our first day in the field, which involved distributing clothes collected by a charitable post office in Rhode Island. Jeff took off early for Karada. I went to Sadr City—a huge Shiite slum in the northeastern corner of Baghdad—with an Iraqi we called Mr. Mustachio, a forty-something man who didn't speak English.

Mr. Mustachio had dark-brown slivers for eyes and an enormous tsunami of black, sculpted mustache. He'd started his own NGO to distribute aid to children in Sadr City. People I trusted vouched for him, so I decided to go along on one of his runs.

Early that morning, I walked through the al-Rashid Gate to his car, a small white station wagon with three of Mr. Mustachio's friends huddled inside. We traversed Baghdad, and it took only a 40-minute drive to see that the city we worked in—the Green Zone bubble of palaces, restaurants, and fortifications—was not anything like what most Iraqis experienced. Sadr City was an utterly Third World hellhole that squeezed the population of Brooklyn, 2.5 million people, into eight square miles, an area about one-tenth the borough's size.

As we drove into town, dust filled the air and donkeys munched piles of still-smoldering trash. Young men in stained athletic wear smoked cigarettes and stared at us. Women dressed head to foot in black abayas were orbited by swarms of children. Freelance Iraqi militiamen in black uniforms held Kalashnikovs, guarding mosque gates and patrolling the streets. Every roadside shop sold something dirty, old, or rotten.

There was no sign of the U.S. military or the official Iraqi police. There were a lot of posters of black-turbaned Shiite clerics. The gray-bearded face of Iraq's most powerful man, Grand Ayatollah Ali al-Sistani, was ever present. But the most popular poster depicted a white-bearded man, Grand Ayatollah Muhammad Sadiq al-Sadr, the populist cleric who'd been assassinated in 1999, allegedly on orders given by Saddam Hussein. Another featured Muhammad Sadiq's son, the young, black-bearded Muqtada al-Sadr.

Around Sadr City, Muqtada was emerging as the Shiite version of Che Guevara. His defiant, iconoclastic attitude was striking a chord with the neighborhood's surplus of young and disenfranchised Shiite men. Already, his meteoric rise was becoming one of the wild cards of the occupation, and it could be linked directly to the country's demographics.

Iraq is a nation of some 25 million people, with a Shiite majority that was long oppressed by the Sunni Arabs who were in power under Saddam. As is true in many Arab and Third World countries, better than half the population is under 20, and Sadr City itself is jammed with young, poor, frustrated men. In the postwar power vacuum that was Iraq under the CPA, Muqtada filled the paternal role in Saddam's collapsed state, and he began to cultivate a giant pool of discontented followers awaiting direction. He started speaking out against the Americans just after the fall of Saddam. In mosques, on the radio, in newspapers, he gave poor Iraqis a tangible enemy, answering the question of why life under the Americans wasn't getting much better.

Of course, the U.S. didn't intend to be an enemy of the Shiites—if anything, the occupation forces wanted to align with them as a way to offset the Sunnis. But the gulf between CPA intentions and Shiite reality was attractive to Muqtada, a man with a thirst for leadership, especially since that gulf was filled with hopeless people looking for a point to their lives.