It has always been absurd to speak of men "dressed in police uniforms travelling in police vehicles" as if they were gunmen in disguise. "Of course they have the uniforms and the vehicles, because they are real policemen," said an Iraqi minister after a similar operation in which 150 people were abducted from the Ministry of Higher Education in the capital last year.

The unit that carried out this kidnapping is almost certainly Shia and is probably under the control of the Mehdi Army or the Badr Organisation. The Finance Ministry in East Baghdad is in a heavily Shia district not far from the Oil and Interior Ministries. There are many checkpoints here, so it would be difficult for a detachment of Sunni insurgents to pass undetected.

The motive is political: Commercial kidnappers in Baghdad - numerous, violent and well-organised though they are - have never had the need or capacity to operate on this scale. The raid also shows good intelligence and a carefully worked-out plan to enter and leave the ministry.

The most obvious explanation for the abductions is that they werein retaliation for the killing of Abu Qader, also known as Wissam Wiali, the Mehdi Army commander in Basra, by a British-backed operation last week. It may be designed to send a message that any British action will be met with retaliation.

The other militia units capable of conducting a raid like this are police and police commandos under the control of that Badr Organisation, the military wing of the Supreme Islamic Iraqi Council (SIIC), whose men still largely run the Interior Ministry. Although it is the Mehdi Army that is invariably singled out for criticism by US and British leaders, the Badr Organisation played a central role in carrying out sectarian killings of Sunnis in 2005 and 2006.

The third suspects in mass abductions against US and British personnel in Iraq are the Iranian-run units that certainly exist. Iranian-inspired retaliatory operations in Iraq appear to have increased since five of their officials were abducted in a US helicopter raid on 11 January on the Kurdish capital of Arbil.

The abductions at the Finance Ministry underline another truth about Iraq. In Arab Iraq, the US and Britain have no allies. For four years the Sunni community has been in rebellion. But the Iraqi Shia only supported the US-led occupation as a means to an end, by which they would legally take power through elections. The Shia do not, at the end of the day, intend to share power with foreign occupiers.

One reason why so many foreign security contractors are employed in Iraq, at vast expense, is that the US, Britain and the Iraqi governments recognise they dare not rely on Iraqis to protect them.