By Adam Brookes
BBC News, Baghdad
Streets in Iraq's busy port city are deserted because of the violence
The role of the militias in Basra is complex.
The largest among them are attached to political parties, with elected representatives.
Those parties - using the militias as muscle - vie for influence and resources.
Basra is fertile ground for both. It is a port city, full of commerce and trade and provides Iraq with access to the sea.
In the surrounding area are many of Iraq's oil refineries.
There is money here, and opportunity. Control Basra and you are a big player in Iraq's future.
The most influential of the militias is the Mehdi Army.
It is the armed wing of a political movement led by the cleric Moqtada Sadr, who has built support among poor, marginalised Shia Muslims.
His followers, we are told, are influential in electricity production, the health authorities, and among the lower ranks in the security forces.
BASRA KEY FACTS
Third largest city, population 2.6 million approx
Located on the Shatt al-Arab waterway leading to the Gulf - making it a centre for commerce and oil exports
Region around city has substantial oil resources
4,000 UK troops based at international airport
And it is this militia which is putting up the strongest resistance to the current military operation.
The movement believes it is being deliberately weakened before local elections scheduled for later this year.
It has been observing a ceasefire - which the army operation in Basra is now testing.
Then there are the Badr Brigades - linked to the political party the Supreme Islamic Council in Iraq, closer to Prime Minister Nouri Maliki, and politically very influential.
They are believed to run the interior ministry with its security and intelligence resources.
The oil facilities around Basra, we are told, come under the influence of a smaller group called Fadhila. The governor of Basra province is a party member.
And then smaller factions and splinter groups complicate the picture ever further.
Mr Maliki's objectives in launching this operation are difficult to discern. On the surface, it is an attempt to reduce militia influence in Basra.
But it is very difficult to see how or why the militias would willingly give up their arms, as Mr Maliki has demanded.
It also seems to be a direct attempt to weaken the Sadrist movement, whose leaders are among Mr Maliki's chief antagonists.
It is a fight for economic resources.
And above all it is a huge and high risk test of the credibility of Iraq's armed forces.