The American military has depended on private contractors since sutlers sold paper, bacon, sugar, and other small luxuries to Continental Army troops during the Revolutionary War.

But the scale of the use of contractors in Iraq is unprecedented in US history, according to a new congressional report that may be the most thorough official account yet of the practice.

As of early 2008, at least 190,000 private personnel were working on US-funded projects in the Iraq theater, the Congressional Budget Office (CBO) survey found. That means that for each uniformed member of the US military in the region, there was also a contract employee – a ratio of 1 to 1.

"It is ... exceptional the degree to which the military's currently relying on such contractors," said CBO director Peter Orszag at an Aug. 12 press conference.

In the Korean conflict, the ratio was 2.5 uniformed personnel for each contractor. In Vietnam, the comparable figure was 5 to 1.

The Balkans conflict of the 1990s provided a glimpse of the future, as it also featured a 1-to-1 military-to-civilian worker ratio.

But in the Balkans, the overall deployment numbers "were of a much smaller scale than what we are seeing in Iraq," Mr. Orszag said.

A number of factors are behind the Pentagon's growing dependence on contractors, says the CBO report. Reductions in the size of the post-cold war military mean that private firms now provide more and more of the logistical support needed to keep the armed services running, such as food supply and housekeeping services on bases. In general, all US agencies in recent decades have outsourced more and more functions judged not inherently governmental.

In Iraq in particular, the ranks of contractors have been bolstered by the US decision to try to rebuild the country while hostilities were still under way.

The CBO estimates the total cost of these military contractor operations from 2003 through 2008 to be $100 billion. That's about 20 percent of all US funding for operations in Iraq.

Most of this money went for logistics support – food-service operation, fuel distribution, equipment maintenance, and procurement and property management.

Roughly $12 billion of the $100 billion total paid for private security contractors – the gun-toting guards of Blackwater and other paramilitary personnel providers.

The CBO looked at the cost of hiring private guards versus the cost of providing similar security with US military units. Among the factors analysts took into consideration was that the Pentagon must pay and outfit multiple brigades to keep one in Iraq, due to deployment rotations.

The result was a tie, according to the CBO.

"The cost of having an Army contingent provide the same services as Blackwater appears to be roughly the same as the cost of the contract itself," Orszag said.

The same holds true for more mundane logistical operations, says the CBO. Hiring a private oil-truck driver for Iraq costs about as much as recruiting, training, and providing a uniformed equivalent.

However, critics of military outsourcing say the real problem is flexibility and command-and-control over private workers.

For instance, private guards have been loose cannons in Iraq, critics say. A federal grand jury is investigating whether Blackwater guards acted illegally when they opened fire in a busy Baghdad intersection last September. Among the most contentious issues in the status-of-forces agreement now being negotiated by the US and Iraqi governments is whether private guards will be subject to arrest and trial by Iraqi authorities.

"One of the key questions surrounding the government's escalating use of military contractors is actually not whether they save the government client money or not.... Rather, the crucial question that should be asked at the onset of any potential outsourcing is simple: Should the task be done by a private company in the first place?" wrote Peter Singer, director of the 21st Century Defense Initiative at the Brookings Institution, in an analysis earlier this year.