The vast slum on the northern edge of Baghdad is often called Sadr City, but its real name is Thawra, which means "revolution" in Arabic. Despite the symbolism of the holy city of Najaf, about 100 miles to the south, it is from Thawra that Moqtada al-Sadr and his Mah-di militia draw most of their support, and where any final showdown with the cleric will take place. Al-Sadr, whose whereabouts were unknown for a time during the fighting in Najaf earlier this month, had almost certainly escaped to Thawra before returning to Najaf when a one-day ceasefire allowed for negotiations.
Initially built in the late 1950s to house farmers who flocked to Baghdad looking for work, Thawra has become increasingly the landing point of the dispossessed, particularly the poor Shia displaced when Saddam Hussein built dams that drained southern marshes in the early 1990s. When I was last in Thawra, on 7 August, Friday prayers had had to be moved because the street in which they are normally held was full of sewage. Children often swim in lakes of the stuff because the trucks that once cleaned up the sumps were lost in the looting after the invasion. Much of the area consists of houses like rabbit warrens. Their doors open on to narrow alleyways, but the main streets are six- and eight-lane boulevards, designed to allow the Iraqi military to enter the slum and put down rebellions. Saddam's security forces tortured, jailed and killed thousands, but could only contain, not eradicate, the al-Sadr resistance (then led by al-Sadr's father, Mohamed Sadeq al-Sadr). The US military failed even to enforce a curfew it imposed on the district earlier this month.
When I went to lunch in the home of some militiamen, they showed me their array of armaments: anti-tank and anti-personnel mines, 160mm mortar rounds, hand grenades, all apparently looted from Iraqi army ammo dumps that the US military had failed to secure after the invasion.
April and May, the months of the first al-Sadr intifada, gave a glimpse of what an assault on Thawra might entail - nearly 1,000 civilians were killed as US troops fought the Mahdi in what were essentially street battles. The Mahdi took disproportionately heavy casualties, but they will become more adept at fighting. Moreover, they are now receiving arms from the Sunni resistance based in the town of Fallujah, 30 miles west of Baghdad, in return for the help they gave the mujahedin in the Fallujah battles against US troops in April.
As I left Thawra, after lunch, I saw men digging holes in the streets, planting anti-tank mines and then paving them over. When US ground troops try to enter the maze-like streets where the militiamen are based - streets impassable to tanks or armoured personnel carriers and, in many cases, even to Humvees - they will risk encirclement. The Israeli military faces a similar problem in Palestinian refugee camps - and it often resorts to bombing from the air or bulldozing homes.
Derided originally as a too-young cleric riding the coat-tails of his father, al-Sadr has become Iraq's most popular politician. The US-led occupation administration helped him. When it closed his newspaper in March, a man whose followers had burned down liquor stores and forced women to veil themselves became overnight a resistance celebrity, admonishing the occupiers for suppressing free speech. He will not go down without a long and terrible fight.
David Enders is former editor of the Baghdad Bulletin. His book on Iraq comes out next spring from University of Michigan Press