BY IRAQ'S recent bloody standards, the violence this week in Basra, the country's second city and its southern capital, was small potatoes. A few protesters were shot dead and a police barracks stormed by British troops after Iraqi police arrested, and then refused to release, two of their comrades. Yet it revealed, once again, the alarming potential for chaos in one of the country's most peaceable areas—which, all the more worryingly, happens to be a heartland of the country's Shia rulers and the repository of most of Iraq's oil. For America's more embattled troops, under fire in the Sunni areas farther north, it will be little consolation to know that if, as is often said, the British are better-loved by the locals, the love is quickly lost.

Tensions started boiling on September 18th, when British soldiers arrested the Basra boss of the Mahdi Army, a militia loyal to Muqtada al-Sadr, a fiery Shia nationalist, who twice last year set southern Iraq ablaze. They accused him of organising roadside bombs that have killed nine people, including two British soldiers, in the past two months: a sign perhaps, that Mr Sadr's people are rethinking their always-weak support for the democratic process. This was also signalled by a day of violent clashes across the south after the unveiling of a new draft constitution last month, between Mr Sadr's lot and members of another Shia militia, the Badr brigade, which is backed by Iran and loyal to the dominant party in Iraq's government, the Supreme Council for Islamic Revolution in Iraq (SCIRI). After the arrest in Basra, several hundred Sadr supporters took to the streets, spoiling for a fight.

To their undoubted delight, the next day, Iraqi traffic policemen arrested two British special-forces soldiers in disguise, suspecting that they were spies. Having spread the rumour across the city—and saying they were spying for Israel—the Sadrists persuaded the police to hand the soldiers over to them. British troops then drove an armoured vehicle through the perimeter wall of the police station where the hapless pair had been held, and later sprung them from the private house where they had been taken. A handful of Sadrists were killed in protests, which went on through the week; the British consulate in Basra came under rocket attack for the first time in several months. According to a Basra teacher: “Whether the prisoners were Israeli terrorists or just a bad version of Lawrence of Arabia, the British have shown their disdain for justice.”

Given last year's violence, the altercation was not surprising. Nor was the British willingness to use muscle: as Britain's death toll in Iraq rises towards 100, its troops are much less likely to assume the “soft posture”, doffing helmets and donning berets, that won them praise in the occupation's early days. Not that this is likely to intimidate their foes: with only 8,500 troops, plus a few hundred Italians, Australians and other allies under their command, the British are too few to secure southern Iraq. Nor are the Iraqi Security Forces (ISF) they have trained, mostly several thousand police, up to the task. Peace prevails in the south, as this latest episode suggests, at the Islamist militias' decree.

According to Basra's previous police chief, barely a quarter of the city's police are dependable. Most are probably moderate—and they do a reasonable and improved job of combating petty crime. But they will not stand against the militiamen in their ranks, who are a very bad lot indeed. At least two journalists, one American and one Iraqi, have been murdered in Basra in recent weeks: both were investigating the militias' activities, which include running protection rackets and assassinating rivals, before their bodies are dumped in a rubbish tip on the edge of the city known as “the lot”. When not thus engaged, the militias rigorously enforce Islamist strictures, beating up women who show an ankle or attacking students enjoying an innocent picnic. A British officer struggled to sound upbeat this week. “There's an unstable sort of stability in Basra,” he said.

The city's governor, Muhammad al-Waili, does not paint a much happier picture. He condemned Britain's actions as “savage, barbaric and irresponsible.” Mr Waili is a member of Fadhila, an Islamist party that fell out with Mr Sadr and has close ties with the ayatollahs in Iran's holy city of Qom.

Badrists v Sadrists

Yet Basra may be the most tightly controlled of the four southern “British” provinces. Of the rest, Muthanna, home to a small Australian garrison, is fairly tranquil, but dominated by the local officials of SCIRI. Farther east, much of Maysan, bordering Iran, including the city of Amara, near where six British military policemen were killed in 2003, is out-of-bounds to British troops; SCIRI's Badr Brigades and the Sadrists vie for control of Maysan as well as neighbouring Dhi Qar province, especially its main town of Nasiriya, where a small garrison of Italian troops is loth to leave its base.

America's government, and many in Iraq's also, like to see Iran's hand in all this. Donald Rumsfeld, the defence secretary, this week warned Iran not to meddle in Basra. And it does seem likely that individuals in all of Basra's main factions are taking the Iranian rial. Though they spend a lot of time squabbling, Basra's four main parties are all dominated by conservative Shias who would like an Islamist state modelled on Iran. To some extent, each party also sees the British as rivals for power, and wants to see them go.

They may get their wish fairly soon. Britain hopes to quit Muthanna, and perhaps Maysan, this year, and Dhi Qar and Basra next year. As this week's violence shows, it will not be a glorious exit: nowhere in southern Iraq is the central government in firm control. Yet with so few forces to control so vast and vexed a region, the British have never pretended to be doing much more than paper over the cracks.

For their American allies, more is at stake. They are increasingly pushing the better units of the 180,000-odd, mostly American-trained, Iraqi forces to the fore in the counter-insurgency, and with some success. Iraqi troops involved in the attack on the northern town of Tal Afar this month, combining 8,500 Americans and Iraqis, fought much better than those given a rearguard role in the assault on Fallujah last November. But top-class ISF units are still few. “It's certainly our goal that in 2006 the Iraqis are out in front in counter-insurgency operations,” says General John Abizaid, chief of America's central command. “However, I can't tell you that we will achieve that goal.”