The fighting in Basra, Baghdad and across Iraq's south is the first full-scale battle of Iraq's long-feared Shia civil war, which has caused hundreds of deaths and destroyed the hard-won calm of the past six months. But many in the coalition believe that the “defining moment in Iraq's history”, as it was described by President Bush, may have started by accident. Coalition officials dismayed by the dismal performance of the Iraqi forces they trained, and by the bold but misguided leadership of Nouri al-Maliki, the Prime Minister, appeared as confused as everybody else about the timing.

Some have hinted that the battle may have been triggered purely by chance, as each side manoeuvred for future control of Basra, the gateway to Iraq's vast, but largely untapped, oil wealth. Several coalition officials said that they were aware of plans by General Mohan al-Furaiji, the Iraqi Army commander, to launch a campaign against the outlaw militias in Basra, but had been led to believe that it would start in June. When it started abruptly on Tuesday some British officials said that they had received less than 24 hours notice.

Some suggested that General al-Furaiji, an experienced officer not known to share plans with his staff, had been moving forces into place when the al-Mahdi Army, which has infiltrated the police and army, were tipped off. That would explain why the plan went off half-cocked, leaving Mr al-Maliki bogged down in a seemingly unwinnable showdown with the al-Mahdi Army militia that is nominally controlled by Shia cleric Hojatoleslam Moqtada al-Sadr. Neither side can afford to back down. For Mr al-Maliki, it would be political suicide, bowing to the rule of militias and clearing the way for the al-Mahdi Army to fulfill its goal of emulating Lebanon's Hezbollah militia. For Hojatoleslam al-Sadr, it would signify an end to his potent mix of anti-American demagoguery backed up by participation in parliament and maintaining a powerful grassroots militia that the US wants to see dissolved as quickly as possible.

In the tangled web of Shia power politics, where a plethora of militias, criminal gangs and Islamic parties vie for control of the oil-rich south, a confluence of US, Iranian and national interests is being played out in the fog of war. Hojatoleslam al-Sadr, who is believed to be in the Iranian shrine city of Qom, has only a loose control of his diffuse al-Mahdi Army, which had carried out a sectarian war with Sunni insurgents before declaring a ceasefire last August.

Joost Hilterman, an Iraq analyst for the International Crisis Group, said that he believed a visit by Dick Cheney, the US Vice-President, more than a week ago may also have helped to trigger the latest crisis. Mr Cheney, keen to point to some sign of political progress to back the security gains of last year's US surge, pushed the Government to sign a law to provide for provincial elections in the autumn.

But that was a “slap in the face” for the Supreme Council, seen by many as having come to power on the back of the US invasion. It lacks popular support in Iraq, and claimed many of its local government seats only because the anti-American Sadrists boycotted the last elections.

The Supreme Council may have urged Mr al-Maliki to push ahead with the operation to undermine the Sadrists, who wield power in part because of their grassroots militia spread across the south and centre of Iraq. But the new Iraqi Army, trained by the US and Britain, fared badly against militiamen fighting on their own turf, often with heavier weapons and with a religious fervour lacking among many policemen and soldiers. Only when US and British forces started to weigh in did Hojatoleslam al-Sadr agree to a ceasefire, although it was unclear if his militia would comply.

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