I recall that the Italian city states of the Renaissance had shifted from using citizen armies to mercenaries over time, as they became the fiefdom’s of narrower interests. Michiavelli thought they helped undermine republican liberty. Lindsay Beyerstein in In These Times:
Private security contractors outnumber now outnumber uniformed military personnel in Iraq. According to Singer, there are 160,000 armed civilian contractors in the country today. Some are under contract to U.S. federal agencies, including the State Department. Others are hired by private interests because the American occupying force cannot maintain adequate security.
Private security companies claim to support the U.S. military, but in fact, the interests of the two groups frequently conflict. Contractors answer to their clients, not to the generals on the ground, and certainly not to the public. Contractors are hired to protect the individuals who sign their checks—even if they undermine the overall counterinsurgency effort in the process.
These unaccountable private forces are actively undermining the U.S. mission in Iraq by inflaming popular opinion against the occupation. According to Singer, Iraqi civilians see security contractors as an extension of the U.S. military.
The more damage the contractors do, the more troops are needed to fight the insurgency. As the need for boots on the ground increases, so does the temptation to hire more contractors.