Well, it's not as if we weren't warned; the Iraqi government had been threatening to move against Fadhila in Umm Qasr, and there had been increasing tension between the Iraqi government and the Sadr movement going back to Christmas. Not so long ago, there were demonstrations in Sadr City against Sadr; they thought the movement wasn't standing up to increasing provocation from police/SCIRI as was//Badr Corps men feeling braver now they didn't have to fight NOIA any more.
You can read the violence in a number of ways; the government/ISCI/Dawa probably briefed it to the Americans as an extension of their counter-insurgency plan to the deep south, with the added twist that this was an operation the Iraqi army would throw all by itself, hence good politics. Sadr of course will consider it an outrage by the collaborationist-Iranian bastards, eerily mirroring Petraeus's response to the Green Zone bombardment; if you adopt Jamie Kenny's policy of trying to think like Leonardo Sciascia, you'll see it merely as a fight for oil rake-offs between (as Douglas Adams put it) rival police gangs. As always, SF leads the way into history.
Daniel Davies has apparently finally taken my much repeated advice and read A Bright Shining Lie, which has apparently led him to conclude that the Dawa-Sadr fighting is a good thing on the grounds that it strengthens the government, even if only as the biggest gang. Well, it has led the annoying look-at-me contrarian Daniel Davies to do so; what the real one thinks I don't know. I don't agree; the Sadr movement demonstrated its deterrent capability on day one, when it resumed rocketing the Green Zone and seized police stations across the Big Gap in southern Iraq, as well as the road between Amara and Basra, rather as they did in the first and second Shia risings in 2004. Further to its massive popularity, the Sadrists also have had at least a tacit alliance with some currents in NOIA - there's a risk of the whole shithouse crashing down. Note that the Dawa and Sadrists, and ISIC, are on the opposite sides of one of Iraq's worst territorial fights.
So inevitably, the US authorities seem to have swallowed the "southern surge" thing, and are now pressing for more British troops to be sent - not just that, but for an advance back into Basra. This is genuinely bugfuck insane and the Prime Minister has no choice but to reject it; there is literally no-one left. Army planners are already looking at calling out at least 2 TA battalions in their entirety to cover routine tasks; a mass of resources is going into Afghanistan; there is some question as to whether there is another brigade in the tubes for the next but one rotation in Iraq. The inter-allied shit just hit the fan.
Of course, nothing would do more for Gordon Brown's polls than turning the fan right up...it's worth noting that officially, the only support MNDSE is giving this operation is aerial reconnaissance; that could perfectly well be provided from Kuwait. However, maybe not.