With US forces increasingly overstretched, private companies are providing a record number of armed personnel in conflict zones around the world - part of a wider trend towards using private contractors to perform duties once carried out by official military units.
More than 20,000 armed personnel employed by private contractors are estimated to be operating in Iraq alone, making up the second largest foreign armed force in the country after the US.
These "private soldiers" have been operating in effect in a legal limbo, with precious few rules governing their activities. However, a handful of legal cases in the US are beginning to define the legal boundaries under which these companies can operate.
In a North Carolina courtroom the families of four US security contractors killed in Falluja last March are suing their former employer, Blackwater USA, claiming they failed to provide them with the adequate equipment or training needed to defend themselves.
In an incident that sparked a large-scale assault by US marines on the city, the victims were burnt and their corpses dragged through the streets. Two were hung from a bridge.
California Microwave Systems, a unit of Northrop Grumman that provides aerial reconnaissance and surveillance systems for the US government, is being sued for negligence by the families of four employees killed in early 2003, when two of its drug surveillance planes crashed in Colombia. Three more employees are being held as hostages by Colombian rebels.
Northrop Grumman declined to comment on the case, but a spokesman said the company was co-operating closely with the US government to win the release of its employees.
Another US security firm, Custer Battles, is defending itself against charges by former employees that it defrauded the US government by overcharging for services in Iraq. The company had several large contracts in Iraq, including one to defend Baghdad's airport following the US invasion. It did not respond to questions about the case but has previously denied the charges.
Two other US companies - CACI and Titan - are facing a class action lawsuit in California relating to the conduct of employees implicated in the Iraqi prisoner abuse scandal. The companies provided interrogators or translators accused in the lawsuit of having participated in torturing prisoners in Iraq.
CACI dismissed the allegations as "baseless", "unfounded" and "frivolous at minimum". Titan did not respond to calls seeking comment.
Although they differ in their specifics, the cases are "all about figuring out the limits of accountability in the absence of any regulation", said Peter Singer, an expert on private military companies at the Brookings Institution in Washington.
For example, who is responsible if a "private soldier" shoots the wrong person? Where can they be tried? Is the company liable if their employees are hurt or killed? Who is accountable?
Civilian contractors are not subject to military justice and are almost never tried at home for acts committed abroad. The conflict zones in which they operate rarely have a functioning justice system that could try them, either.
In practice, then, contractors have enjoyed broad immunity from prosecution for acts committed abroad.
In the recent Iraq prisoner abuse scandal, for example, US soldiers were quickly court-martialled. By contrast, the civilian contractors from CACI and Titan alleged to have been involved have not yet been subject to any government prosecution.
Charles Graner, the soldier sentenced to 10 years in jail for leading the abuse of prisoners at Abu Ghraib, said in testimony last month that the civilian contractors had played a central role in the abuse. "A lot of the weird stuff came from civilian contractors," he said, referring to the photographs of naked Iraqi prisoners in sexually humiliating situations.
The US Congress passed the Military Extraterritorial Jurisdiction Act in 2000, giving US courts the ability to try contractors working abroad for the defence department.
However, it is so far untested and limited in scope. For example, it does not cover contractors working abroad for the Central Intelligence Agency.
In the case of Custer Battles, the company has argued that it is not subject to US courts because it was paid by the US-led Coalition Provisional Authority using Iraqi oil funds - not US tax money. According to this argument, any money allegedly stolen would have belonged to the Iraqi people. But it looks unlikely that the company could be tried in Iraq. Two days before leaving the country last June, Paul Bremer, the US administrator, signed a decree granting foreign contractors immunity from Iraqi law.
Some of the more established companies working in the sector, such as ArmorGroup, have been calling for some regulation of the sector to help settle questions vital to their business. In the absence of such regulation, those questions could be settled by the courts.
"Other countries develop rules and regulation," Mr Singer said. "We do reform through litigation."
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