What’s behind this sudden internal battle in Basra?
Here’s an interesting analysis from UPI. I have no idea how credible it is, but parts of it make some sense to me.

The violence that has killed dozens and injured hundreds since Tuesday is billed as Iraq’s military against “criminals, terrorist forces and outlaws,” in the words of Maliki. But political parties and their militias have gained a stronghold in Baghdad and Basra, from elected office to the security forces, police and those protecting the oil infrastructure. And the battle is looking more like two leading political parties against two disenfranchised parties, all Shiite Arab.
“It’s an internal Shiite war for who is going to represent the Shiite community in Iraq,” said Kenneth Katzman, a Middle East expert at the Congressional Research Service. The operation was planned a month ago, he said, and the target was the illegal activity like oil smuggling taking place under the control of the Fadhila Party and other armed groups.

Sadr has aspirations, we know that. Maliki is desperate to hang on to his own power. So we have two or more armed factions fighting for control of the biggest oil port and infrastructure in the country.
Sounds like civil war to me.