The Modern Mercenary: Private Armies and What They Mean for World Order. By Sean McFate.Oxford University Press; 248 pages; $29.95 and £19.99. Buy from Amazon.com; Amazon.co.uk

IN THE pirate-infested waters off the coast of Somalia around 180 private military contractors from 35 countries work to protect international shipping. They cost as little as a tenth of the official protection provided by the governments of France, Holland and Spain. Yet they can deploy lethal force and have proved very effective; they have even formed an industry group, the Security Association for the Maritime Industry, to represent their interests.

As Sean McFate shows in a fascinating and disturbing book, “The Modern Mercenary”, the provision of private armies to the world’s conflict zones has boomed in the past 25 years. The market for these firms could evolve from a monopsony, in which the dominant buyer has been the American government, into something more open and competitive. As it does so, he argues, these armies may turn from a force that is mostly for peace into a threat.

The end of the cold war provided the opening for private armies. In 1994 Executive Outcomes, founded by a group drawn from various South African special forces units, offered to help stop the genocide in Rwanda. The United Nations turned the company down. Although it later found work in Angola, Kenya and elsewhere, it closed after the South African government outlawed mercenaries in 1998. The market really took off when America, under George W. Bush, wanted support for its occupations of Afghanistan and Iraq. One firm, Blackwater, stood out, but the work gained a unfavourable reputation and the company has since changed its name twice, first to Xe and now Academi.

The growing faith in free markets and privatisation, ushered in by Ronald Reagan and Margaret Thatcher, and the increasing political need to hide the human cost of war by reducing deaths in the standing army, have encouraged the use of private soldiers. So did the belief that they were often more effective and efficient. (Erik Prince, Blackwater’s founder, liked to describe his firm as the FedEx of the American national security apparatus.)

Mr McFate writes with an insider’s knowledge, having worked for DynCorp, another private military company, on assignments that he says included foiling a plot to assassinate the president of Burundi. On another occasion, he says, he was approached by a famous actress turned humanitarian who, with various human- rights groups, wanted to hire Blackwater to set up safe havens in Sudan to protect civilians fleeing the janjaweed militia. In the end, though, they decided that the risks of an illegal action of this kind outweighed the benefits.

With American demand for private military operators falling as it scales back its overseas operations, Mr McFate expects demand to grow from other customers, including humanitarian organisations and less idealistic groups. He is alarmed by the prospect, not least because he feels that in a truly free market mercenary armies might be encouraged to seek profits by starting new wars.

The author fears that the world is entering an era of “neomedievalism” in which the state loses its monopoly of legal force and instead other wealthy groups or individuals fund private military adventures. Africa, in particular, looks ripe for this, he says. He believes America has missed the opportunity to regulate this market properly, but thinks there is still a chance for the United Nations to do so by making private military companies an important part of its peacekeeping operations, something it so far declines to do.

In the end, though, Mr McFate tends to overstate his case. Private armies may indeed play a role in failed and failing states, but it is unlikely that modern mercenaries will become more important than the standing armies of NATO or China soon. The author’s conclusion that “international relations in the 21st century will have more in common with the 12th century than with the 20th” sounds like an exaggeration. Yet the worrying trends he describes make this book a powerful call to arms to those who do not want a world awash with mercenaries.