There has been growing concern with the Iraqi government about the disorder in the city. In recent weeks, Lt. Gen. Mohan al-Fireji, a senior Iraqi commander in Basra, proposed that additional forces be sent.

Prompted by this suggestion, a detailed plan was being developed by American and Iraqi officials, which involved the establishment of combat outposts in the city and the deployment of Iraqi SWAT teams, Iraqi Special Forces and Interior Ministry units, as well as Iraqi brigades.

That plan was the subject of a March 21 evening meeting that Gen. David H. Petraeus, the senior American commander in Iraq, convened with Mowaffak al-Rubaie, Mr. Maliki’s national security adviser. At the end of that session, General Petraeus was asked to meet with Mr. Maliki the next morning. The prime minister, it seemed, had his own ideas on how to deal with Basra and planned to travel to the city to oversee the implementation of his plan.

“Effectively, much of the city was under militia control and had been for some time,” Mr. Crocker said. “Maliki kept hearing this along with some pretty graphic descriptions of militia excesses and just decided, ‘I am going to go down there and take care of this.’ I think for him it was a Karbala moment.” Last August, Mr. Maliki rushed to Karbala after an outbreak of Shiite-on-Shiite violence, fired the police commander and oversaw the successful effort to restore order to the city.

One American intelligence officer in Washington, however, had a somewhat different interpretation of the prime minister’s motivations. While restoring order was his stated goal, he asserted, the Iraqi leader was also eager to weaken the Mahdi Army and the affiliated political party of the renegade cleric Moktada al-Sadr before provincial elections in the south that are expected to be to be held this year. The Islamic Supreme Council of Iraq, a Shiite political party and militia that are rivals to Mr. Sadr, his party and his militia, form a crucial part of Mr. Maliki’s political coalition.

When Mr. Maliki met with General Petraeus on the morning of March 22, he indicated that his goal was to take on the “criminals and gang leaders” in Basra, according to an account of the meeting by an American official. Mr. Maliki explained that the operation would be an Iraqi affair but that he might need air support from the Americans.

Image MARCH 30 Mahdi Army fighters as they stormed a state-run television center in Basra, forcing the Iraqi military guards surrounding the building to flee and setting armored vehicles on fire. Credit... Nabil al-Jurani/Associated Press

He said that he was going to meet with sheiks, religious figures and other local leaders, taking advantage of the additional leverage he hoped to gain by sending in troops, fostering economic development programs and sending teams of judges to try to punish corrupt and violent behavior.