Quis custodiet ipsos custodes? Who will guard the guardians? Or, when discussing the conduct in Iraq of America's largest and most notorious private security contractor, Blackwater, who will protect us from the protectors? The ancient question raises the most profound issues of power and the accountability of power. But as far as Blackwater goes, the shocking and deeply scary answer - at least until now - has been: nobody.
In Iraq, Blackwater's most important task is to protect State Department personnel in the central region around Baghdad. Over the past couple of years it has been paid $832m (£407m) to do the job, and in one sense has performed impeccably. Though dozens of its personnel have been killed, not a single American diplomat has been lost. But how this 100 per cent record has been achieved is quite another matter.
In Iraq, Blackwater in practice is subject to no laws, local or American. Its 1,000-odd men there (most of them former soldiers) amount to a small mercenary army in the service of the United States. Like most mercenaries, they are pretty effective. But as mercenaries their loyalty lies only to their job, their employer and themselves. When there are no laws to answer to, anything goes - and on 16 September the suspicion is that it did, as a Blackwater convoy is accused of recklessly opening fire in central Baghdad, killing at least 14 unarmed Iraqi civilians.
Blackwater says its men were attacked first. Eyewitness accounts, the Iraqi government and now a report from the US military beg to differ. The incident has become yet another moral, diplomatic and public relations disaster for the US in Iraq. For the local populace, it has further undermined what trust remained in the American mission in their country. Once again, lofty talk about freedom, democracy and human rights comes across as rank hypocrisy.
A congressional report last week found that since January 2005, Blackwater has been involved in almost 200 shooting incidents. In four out of five of them, its employees had fired first. At best, it suggests, the State Department exercised no supervision of Blackwater operations, and at worst was complicit in a cover-up. The only sanctions were imposed by the company itself - fines, or in extreme cases dismissal. Last December, a drunken Blackwater employee shot dead a bodyguard of one of Iraq's vice-presidents. He was sent back to the US and sacked.
Blackwater paid just $15,000, after a US official said that a larger payment would lead to "people trying to get killed by our guys to financially guarantee their family's future". So much for caring diplomacy.
Blackwater is merely an extreme example of the rush to privatise. The process was well under way in the Clinton years, before the arrival of a Republican President convinced that the private sector is automatically superior to the public one. Health-care management, student loans and the US prison system have all been "outsourced" in varying degrees, to supposedly more efficient private operators. So why not the Pentagon?
In Iraq there was little choice in the matter. Without Blackwater and the other private security firms to protect American diplomats, the overstretched US military could not function. Between 20,000 and 30,000 armed security "contractors" are reckoned to be in Iraq, equal to between a fifth and a quarter of the total pre-surge strength of official US forces. Throw in non-military contractors - catering, cleaning, maintenance staff, etc - and the figure jumps to some 130,000. By one Democratic congressman's estimate, about 40 cents in every dollar spent on the occupation goes to outside contractors.
Complaints about the trigger-happy, arrogant and occasionally murderous behaviour of Blackwater, in particular, have been around for years, ever since Paul Bremer, then US proconsul as head of the Coalition Provisional Authority, decreed in 2004 that private security forces would be outside Iraqi jurisdiction.
Last week Erik Prince, Blackwater's publicity-shy founder and owner, was hauled before Congress to insist that his men were not cowboy mercenaries but patriotic Americans. The FBI, it was announced, would head an investigation into the incident. On Friday, the State Department announced steps to prevent such massacres of the innocents in the future. Diplomatic "agents" will travel with each Blackwater convoy, a "black box" will provide an electronic record of controversial incidents, and there will be closer liaison between Blackwater and the local US military commanders. Finally, Congress moved towards passage of a law making Blackwater personnel subject, if not to Iraqi laws, then at least to those of the country that hired them.
Last year Blackwater's vice-chairman, Cofer Black, reportedly indicated the company was ready to offer brigade-sized units - several thousand men - as part of its services. Who will make sure such a force does not subvert government, or carry out the state's dirty business, beyond the normal chain of command and any semblance of scrutiny?
Or, to put it another way, who will protect us from our protectors?