By Paul Wood
Defence correspondent, BBC News
Last November, an official assessment by the Pentagon judged Basra "not ready for transition" to control by the Iraqi security forces.
Iraqi and British security have jointly raided rogue security facilities
Only one other province had such severe problems, Anbar, home to the insurgent strong-holds of Falluja and Ramadi.
Basra was in an even worse state than Baghdad, according to the report submitted by the US military to Congress, entitled Measuring Stability and Security in Iraq.
That may, in part, have explained the lukewarm US reception to the British announcement a mere six weeks later, in January, of a "draw-down" in Basra.
The British government said the Army's main bases in the city would be closed and the total British strength reduced by several thousand over time.
The theory behind this is that the Iraqi forces are now ready to take over. The raids over the weekend were indeed led by the Iraqi security forces - but targeted other parts of the Iraqi security forces.
'Ethnic cleansing by stealth'
At the National Iraqi Intelligence Agency detention facility in central Basra, some 37 prisoners were found, some bearing signs of torture.
The aim of the raid had been to detain a "known death squad leader" - the implication being that the death squads are often official in character.
Ordinary Iraqis know this. When kidnappings or drive-by shootings are carried out "by men in police uniforms" that is because often it is the police.
What happened after the Basra raid, though, was perhaps even more disturbing than the thought that many Iraqi police are in the business of murder.
Instead of congratulating the special forces units involved in the raid, the Iraqi prime minister called for an investigation into what, he implied, were British misdeeds.
The National Iraqi Intelligence Agency HQ was raided on Sunday
"The prime minister... affirmed the need to punish those who have carried out this unlawful and irresponsible act," said a statement from his office.
This is not the first time, even in recent weeks, that British troops have had to carry out such "unlawful and irresponsible acts" - that is go into a police station to stop torture and murder.
In December, British and Iraqi troops assaulted a police station in Basra and rescued 127 prisoners from squalid conditions. Many prisoners had been beaten and otherwise abused.
Electric drills are used as an instrument of torture by the most feared of the rogue police officers.
Sunni and Christian residents of Basra have complained of ethnic cleansing by stealth in Iraq's second city.
Basra power struggle
This is part of a nationwide pattern. Last year, US and Iraqi forces discovered a secret prison in Baghdad run by the Shia-controlled interior ministry.
Known as "Site 4", about 1,400 prisoners were crammed together there and subjected to systematic abuse.
In Basra, the Iraqi special forces said they had sought permission from Baghdad to carry out the weekend's raid. No-one seems to have told the prime minister.
One question is whether Prime Minister Nouri Maliki's Shia power base includes people who are too close to some of the militias.
If so, he will struggle to take effective action against them.
So now the worrying prospect is that along with the Sunni-Shia violence, the intra-Shia conflict, and the Sunni and Shia insurgent attacks on the coalition, there may be a struggle between different branches of the security forces in Basra, and with Baghdad, over who is in charge.
All of this is one reason why the announcement of a British draw-down was actually less ambitious than the government and the military had originally intended.
The plan had been to cut the British force by half in May. This will now take until November, or longer.
The Pentagon's next quarterly report to Congress on security in Iraq's different provinces is due shortly.
It will be interesting to see if Basra, along with Iraq's most bloody province, Anbar, is still coloured red, still judged not ready to be handed over to the local security forces.