By Hugh Sykes
BBC News, Baghdad
In an aerial photograph of Baghdad there are two prominent features - the River Tigris and Sadr City.
The Tigris is a wide, slow moving ribbon of water that curls through the centre of the Iraqi capital.
Sadr City is a substantial, almost perfectly square densely populated grid-pattern suburb that looks as if it has been bolted onto the north-east of Baghdad.
It did not grow organically like the rest of the capital - it was built in the late 1950s to help solve a housing shortage.
It was first named Revolution City - later changed to Saddam City, which was taken as an insult to the Shia who lived there, who were systematically oppressed by the Saddam Hussein regime.
After the fall of Saddam Hussein in 2003, the district was again briefly known as Revolution City.
Moqtada Sadr has strong support in the area
But it was then re-named Sadr City after Ayatollah Mohammed Sadek al-Sadr, a revered cleric who - along with two of his sons - was murdered on the orders of the former Iraqi leader.
At least three million people live in Sadr City, almost all of them Shia Muslims.
It is now the stronghold of one of the Ayatollah's two surviving sons, Moqtada Sadr, whose political headquarters are in a fortified building on one of the main avenues that run through the area.
Some Baghdadis refer to Sadr City residents as "shororooghi", a derogatory term that is intended to suggest ignorance and lack of education.
Sadr City is, in effect, a Shia township.
It is physically, and psychologically separated from Baghdad by the wide Army Canal and a busy highway that runs alongside it.
Many in Sadr City look to the Mehdi Army, not the US, for security
And, like a South African township, it is not a homogeneous place - there are overcrowded slum districts with raw sewage overflowing into the street, and smarter neighbourhoods with larger houses and better conditions.
Some areas of Sadr City have been transformed by American military engineers, who have installed entirely new main sewers and more than two dozen reverse-osmosis water-treatment filters in primary schools.
An American colonel who showed me some examples of this undeniably successful reconstruction urged me to ask local people, "who provides your security?", hoping the answer would be "the police" or "the Iraqi Army".
But when I reassured them they could tell the truth, the most common reply was "the Mehdi Army".
Politics and 'protection'
Sadr City is mostly under the control of Moqtada Sadr's Mehdi Army militia - whose activities range from sectarian revenge killings, and attacks on multinational security forces, to community work and neighbourhood-watch.
The militia have also been involved in protection rackets and extortion at some of the very few petrol stations in Sadr City - mostly by simply adding a premium to the official pump price.
Moqtada Sadr is also involved in Iraqi politics at a high level - his party has seats in parliament, and he helps sustain Prime Minister Nouri Maliki's coalition government.
Mr Sadr has recently instructed his followers to stop killing fellow Muslims - whether Sunni or Shia - and to concentrate on resisting the US and British occupation and on repelling the extreme Sunni Wahabists of al-Qaeda in Iraq.
Whether Mr Sadr's followers in Sadr City heed this instruction remains to be seen - but he has threatened rogue Mehdi Army commanders with the wrath of God.
"If you do not obey, you will regret it," announced a preacher who is often a mouthpiece for Mr Sadr, "indeed, I declare that you will be cursed."