The Iraqi government's growing assault on militia forces in Basra had been expected for some time. The actual timing of the assault, however, has reportedly caught the United States by surprise.
That is generally believable except for that the British newspaper the Independent reported on March 20th statements by an Iraqi military leader saying that the Iraqi military were about to do just what they are doing.
That American forces on the ground would be unaware of such statements by the Iraqi general in charge of security for Southern Iraq is unlikely, so why let the story hang out there unanswered?
Two possibilities come to mind: the U.S. was not interested in preventing the action, or they were unable to.
The fact that Moqtada al Sadr, whose forces and allies are facing the brunt of the Iraqi government's attack, has yet to lift his cease-fire order adds an intriguing twist to the calculus.
The decision by the powerful Shiite leader to impose a cease-fire has led to a far quieter period in Iraq than there otherwise would have been. U.S. and Iraqi government forces have been freed up to turn their attention towards the Sunni militias, often in cooperation (tacit or overt) with the Shiite militias they were once battling (Sadr led his forces, the Mahdi army, and others in two uprisings against the U.S. military).
The government assault unraveled fairly quickly, however, and U.S. forces have become increasingly involved in supporting the fledgling military force.
Another aspect to Sadr's refusal to sanction action is he and his allies are direct political competitors with the current leadership of the government, Nuri al-Maliki and his Dawa party as well as the Supreme Islamic Iraqi Council (formerly the Supreme Council for Islamic Revolution in Iraq, SCIRI). They refused to participate in the last round of elections and have made no promises to the next round.
Given that the Shiite forces aligned with Sadr are now being attacked by a government whose legitimacy they do not recognize, their "official" neutrality may show some political cunning. The forces are certainly defending themselves, officially or not, and the government forces are unlikely to achieve any sort of victory barring intense American involvement.
Sadr is far more likely to come out ahead in such a match-up. He is unlikely to have any less friendly relations with the opposition and by staying above the fray even under the government's assault will only improve his image among Shiites sympathetic to him, or distrusting of the Maliki-led government.