Sadr's fighters retaliated for the Basra operation by increasing their rocket attacks on the Green Zone from Sadr City. The United States built a wall far enough into the Sadr City slum so that the militia's rockets couldn't reach the seat of the U.S.-backed government and the U.S. Embassy.
The largest Shiite bloc in government then signed a peace treaty with the Mahdi militia, and the army rolled into the northern portion of Sadr City -- through the U.S.-built wall -- completely uncontested.
Today, the Iraqi military patrols the heart of Sadr City, its first significant presence in the area since the U.S. invasion.
"The people of Sadr City are happy for the agreement between us and the government and support it," Sheik Salman Al-Furaiji, the head of Sadr's bloc inside Sadr City, told ABC News while sitting in his office. "They are happy that the governmental forces are here to bring security."
But the government's forces need to bring more than security. The operation is in part designed to convince Sadr City residents that the soldiers -- and the government that sent them in -- are on their side and are more reliable than the Mahdi Army.
"They are our people and our countrymen and our relatives," Abu Abdullah, an Iraqi officer, told ABC News while patrolling in Sadr City. "We are here to serve the people -- to provide them with services and target the outlaws."
That may be the goal, but some residents haven't had electricity in a week, and it is nearly impossible to quickly fix a grid that is dilapidated.
The operation is also designed to reduce the capabilities of the Mahdi Army for good. The peace treaty allows the Iraqi army to arrest senior Mahdi Army officials and confiscate their heavy weapons.
So far, the Iraqi army has collected 88 roadside bombs; 45 explosively formed penetrators, which have been known to pierce the thickest American armor; and about 200 mortar, artillery and tank rounds. The caches of weapons are routinely shown on Iraqiya television here.
But most of the heavy weapons and the senior leadership are no longer in the city, raising questions about whether the military is able and willing to do any lasting damage to the Mahdi Army.
"A lot of the bad guys, the JAM folks have actually cleared out of the area, taken a lot of their larger weapons most likely and relocated them to wherever," Mount said, referring to the Mahdi Army with an abbreviation of its Arabic name.
And throughout the city, Sadr's influence is always on stark display. His and his father's faces are on every street.
South of the wall, the United States patrols the Jamila market, the second largest in the country and the source of much of Baghdad's fruit, rice, flour and oil.
ABC News walked through the market with soldiers from the Charlie company, 1st battalion of the 6th infantry division this week. A dust storm had settled over the shops, which were all shuttered during the hottest part of the day. The stench of sewage filled the market and sludge was caked into the sidewalks. The military had already hauled out dozens of trucks' worth of garbage, but the market was nowhere near functioning.
"If we can revitalize this area, it will affect more than just Sadr City, more than just the Jamila market, but all of Baghdad," Second Lt. Nathaniel Sparks, walking through the wholesale section of the market, told ABC News.