The dogs of war need a tight leash

The mercenary - the "soldier of fortune" - has been a feature of organised warfare for at least 4,000 years. In other words, for about as long as there has been organised warfare, it has had a private sector. King Shulgi of Ur hired professional soldiers in the 21st century BC, and the pharaoh of the Bible used mercenaries to expel the Israelites. Europe of the 14th century saw the rise of the mercenary companies - the Catalan Grand Company, the Great Company, the White Company - followed by the condottieri . During the decolonisation struggles of the 20th century, the so-called dogs of war assumed celebrity status in the media and in cinemas.

In some cases, mercenaries - seeking a less tiring source of income - ventured into legitimate trade, hence the Dutch and British East India Companies.

For as long as the private military industry has been around, it has at worst been reviled and at best approached with a long spoon. The Vatican Third Lateran Council of 1179 condemned mercenary companies, and the Magna Carta of 1215 called for their expulsion from England. The 1977 Geneva Protocol 1 removed from mercenaries the privileges of international humanitarian law, and in 1989 the United Nations General Assembly adopted a convention against the recruitment, use, financing and training of mercenaries. But since the end of the Cold War, the private military sector has undergone a renaissance. Private military firms (PMFs) are often in the news, and many of them are quoted on stock exchanges. There has even been some hankering for the good old days of the trading companies: a failed mercenary coup against the Government of Equatorial Guinea was reported in early 2005 to have had as its aim the creation of a private fiefdom, to be known as the Bight of Benin Company, modelled on the British East India Company. The private military/ security market is booming: annual revenues are about $100 billion (£55 billion) and are expected to double by 2010.

P.W. Singer's Corporate Warriors is a comprehensive analysis of the political economy of the contemporary private military/ security sector.

Singer calls for an imaginative and intelligent real-world response to an important but increasingly complex and controversial phenomenon.

There are many well-rehearsed concerns about PMFs. They are in business for profit, so their loyalty as military professionals will be to the highest bidder rather than to the noblest cause or the national interest. They could easily switch from a passive/defensive to an active/ offensive role and destabilise governments. They are not bound closely enough by international humanitarian law and their moral status is dubious.

Above all, PMFs undermine our conviction, expressed most clearly by Max Weber, that the only proper use of organised armed force is that authorised exclusively by the government of a state. In a comment that Weberians will find chilling, Singer describes the privatised military industry as representing "alternative patterns of power and authority linked to the global market, rather than limited by the territorial state".

But the greatest difficulty with PMFs is that they are much more than just mercenary bands, and in many cases what they do is by no means all bad.

Singer classifies PMFs either as "providers" (of military power for use on the battlefield), "consultants" (in military training programmes) or "support" firms (offering logistic and maintenance support but not fighting on behalf of the client).

As such, they appeal to national governments that are reluctant to pay and equip vast national defence forces and to international organisations that lack the will and the capacity to use military force where necessary and at the right moment. PMFs can undertake post-conflict activities such as landmine clearance and can assist in retraining armed forces. PMFs can act as "investment enablers", reassuring investors that even in a post-conflict country lacking adequate security forces, vulnerable infrastructure can be protected. PMFs can also protect threatened non-governmental organisations and undertake humanitarian rescues.

Corporate Warriors makes a compelling argument for an urgent reappraisal of the private military sector. PMFs present important challenges to established thinking about the organisation of the international system and the use of armed force within it. These challenges must be answered, probably with some regulation and licensing for which the more far-sighted PMFs have been calling, before the opportunity is lost to reassert political control over this increasingly decisive and influential area of military activity.

Paul Cornish is professor of international security and head of the International Security Programme, Chatham House.