When private military contractors, working for Blackwater, the US security company, shot and killed 11 Iraqis in a Baghdad square on September 16, the incident could easily have disappeared into the grim catalogue of a day’s mayhem in Iraq, like so many similar incidents before it. That it did not testifies not only to the intensity of the backlash against the occupation and the war in both countries, but also to growing awareness of how dependent the US military has become on mercenaries.

The number of US troops in Iraq is now at a peak of 168,000; but they rely on an even bigger army of 180,000 contractors, employed by more than two dozen private companies. Many of these people do the cooking and laundry. But about a quarter of them are fighters.