By Trevor Royle

MILITARY planners call it the tipping point, the decisive moment when circumstances conspire to replace one set of certainties with the precise opposite, when what passes for normality slides into anarchy and the authorities are powerless to do anything to halt the mayhem.

In Iraq this weekend, the expression has taken on a new and alarming significance: following a brief period of hope the country has once more been plunged into a maelstrom of communal violence, with mass demonstrations, killings and gun battles between the security forces and Shi'ite militias.

No one knows for sure if Iraq has reached its own tipping point - as ever in this benighted country, making predictions can never be an exact science - but all the signs suggest that the country is slipping into a new crisis.

This is not just another escalation of violence but a confrontation which could quite easily turn into a civil war between government forces and the rival militia groups, which represent different Shi'ite groupings as well as rogue gangs and other criminal tendencies. So serious is the situation that the US-backed government has extended the time limit until April 8 for the militias to disarm, a sure sign that the original deadline of this weekend was being ignored by the fighters on the streets.

At the end of a strife-torn week the facts speak for themselves.

An outbreak of fighting between Iraqi security forces and the Mehdi Army militias in Basra has left over 100 dead and many more wounded. In Baghdad, a mortar attack in the protected green zone killed three and wounded 15, while thousands of supporters of the Shi-ite cleric Moqtada al-Sadr took to the streets and plunged neighbouring Sadr City into violence, threatening the precarious ceasefire that has held the peace since last autumn.

Hilla, Kut and Diwaniya have also seen sporadic violence, and in the northern city of Kirkuk two peshmarga security officers were killed by a car bomb. Worse followed in Nasiriyah on Friday when firefights between local police forces and Shi'ite militia resulted in scores of casualties dead and wounded.

Perhaps the most damaging incidents occurred on Thursday when rogue militias blew up one of the two main export pipelines near Basra, a move that reduced the country's oil exports by a third and pushed up the price of oil by one US dollar per barrel. Three other pipelines were shut down as a result of the attacks, a disruption which the country can ill afford at a time when it is struggling to make itself less reliant on US aid.

Calls for a political solution to the impasse have been made by Mehdi Army officials, but all have stopped short of demanding a cessation of the violence.

In response to the crisis, the Iraqi government went into emergency session on Friday and ordered the continuation of an earlier three-day curfew in Baghdad. At the same time, prime minister Nuri al-Maliki retained personal control of the operations in Baghdad where the Iraqi 14th Division is currently engaged in the operation against the militias. Maliki's director of operations, major-general Abdul-Aziz Mohammed, told reporters that the fighting was being directed against "criminals and outlaws" but the commanders of the Mehdi Army seem to be operating to a somewhat different agenda.

Their demands are quite simple, if unattainable: Sadr's supporters want Maliki's resignation, the complete withdrawal of foreign troops and an end to the military operations in Basra. To reinforce their point they set fire to the headquarters of Maliki's Dawa Party in the Shaab quarter of northern Baghdad.

"I call on Nuri al-Maliki and the Iraqi army to stand shoulder to shoulder with the Iraqi people and not hurt them," one black-cloaked man told the watching crowds. "We in the Mehdi Army are your brothers. Do not try to ignite situation. The number of people killed is mounting in Basra."

So far the response from the occupying powers has been muted with the official line being that Iraqi security forces are more than capable of taking the lead in engaging the Shi-ite militias, but that does not mean that US and British forces have not been involved.

For the first time in many months coalition war planes have been in action in the skies above Baghdad and Basra and it is clear that they have been flying in support of the forces on the ground. Faced by television evidence that air power had been used on Friday the British military spokesman Major Tom Holloway admitted that there had been "incidences of weapons release" - a novel way of describing an air strike on two Mehdi Army positions.

Behind that bland assurance the British forces in Basra are facing a predicament. After last December's drawdown the garrison now numbers 4,100 soldiers but they are more or less imprisoned within the airport perimeter, a move that has reduced casualties but makes the force look powerless. Officially its task is training the Iraqi security forces while maintaining a policy of "overwatch".

In the first role the British have been acting as mentors to the nascent police and security forces while the second role obliges them to provide a rapid response force should violence spin out of the control of the Iraqis. So far that has not been required but senior British commanders know that they are operating on a knife edge where numbers are too small to make any decisive intervention.

"Practise and long experience have told us that the best way forward is to train up the Iraqis so that they can take up responsibility for their own security issues," argues a British military source. "As long as our guys were on the streets they were simply seen as the oppressor, not a helpful position that. Now e can see if the Iraqis really are up for it and capable of holding the line."

The other problem facing the British garrison is the provision of reliable intelligence about the militias. Although they possess adequate day-to-day intelligence about the movements of the rival gangs it is not always possible to get consistent information about their make-up, allegiances and motives. It is easy to discern the differences between the Mehdi Army and its closest rival the Karbala-based Badr Brigades but the shifting allegiances of the minor groupings are often impossible to discern even for locals who are those closest to the action.

There is also come confusion over the motives of the the Iraqi government in deciding to make an assault on the Shi'ite militias. When Maliki came to power in 2006 his Shi'ite Alliance depended for its majority on the 32 seats provided by Sadr's supporters in parliament. Even though that support was withdrawn in September last year following Maliki's refusal to set a timetable for an end to the coalition's military presence, the Sadr faction remains an important grouping in Iraqi politics.

By common consent the decision to implement a ceasefire helped to reduce violence during the US surge at the end of 2007 and its renewal a few weeks ago was seen as a harbinger of further moves towards normalisation of Iraqi society.

Now the Sadr leadership fears that not only is it being sidelined but it is in danger of being eliminated by an administration which is in league with the US and its allies. In a statement released on Friday Asharq Al-Awsat-Harith al-Adhari, Sadr's chief of staff in Baghdad accused Maliki of using "bone-breaking" methods to crush Sadr's followers ahead of the forthcoming local elections.

"I advised you in previous statements to be patient and respect the orders of the Hawzah Shi-ite seminary," he told Sadr's supporters. "I asked you to stand up to the onslaught by the occupier and his lackeys who are implementing his Maliki's plans that aim to harm the sons of this noble line. Recent events in Basra, Al-Kut, and Al-Sadr City have proved that the Iraqi Government is pressing ahead, in cooperation with the occupation forces, with the implementation of its evil plan and which coincides with the approaching governorates councils' elections for the purpose of distorting the image of Al-Sadr Trend whose supporters are now suffering from continued arrests in all the governorates."

Presumably Maliki's decision was given the go-ahead by the US which has long been suspicious of Sadr's links with the Iranians - last week senior US commanders repeated accusations that the Iranian Republican Guard had been supplying the Shi'ite militias with weapons including the technology for making deadly roadside bombs. If that is the case it was a question of going for broke for although Sadr is viewed as a parvenu in the upper echelons of the Shi-ite community on account of his lack of religious qualifications his movement enjoys widespread popular support amongst young Iraqis and Sadr City is a virtual stronghold and no-go area.

When Sadr agreed to extend the truce in February western intelligence regarded it as a move to retain influence and to bide time while he continued his religious training at a seminary in Qom, Iran to become an ayatollah. At the same time Sadr decided to purge his movement of rogue elements and to re-assert control at a time when a semblance of normality had returned to Basra in the wake of the British drawdown and the realignment of its garrison.

That gave Maliki his chance and last week's crackdown can be seen as a last-ditch attempt to crush the Sadr movement once and for all and in so doing to reimpose Baghdad's authority in a part of Iraq which for too long has been a prey to Shi'ite rivalries and internecine violence.

It is a high risk strategy. While Sadr is a thorn in the flesh of the Shi-ite movement his youth and his unprepossessing appearance belie a steely determination to get his way as he has shown in his dealings with the Supreme Islamic Iraqi Council. Within Sadr City he has given hope to thousands of dispossessed young Iraqis and although most of his influence has been gained by the gun he is also respected as a crafty political tactician.

Maliki must have been confident that he enjoyed superior strength when he launched last week's offensive with up to 55,000 troops. If it succeeds he will have imposed on his government's authority on Basra, a city which was in danger of becoming a fiefdom under Sadr's control. However, if it fails the odds are that Iraq could have taken another and perhaps decisive step towards the abyss of civil war. At the same time Sadr's supporters are also positive that they hold all the aces. "We are very patient," said one of Sadr's principal lieutenants as a dangerous week came to an end. "But if the government does not respond to our demands, something bad will happen."