The single most important change in military affairs in recent history is the unprecedented role private contractors have come to play in modern warfare. This has been a trend in the making from the 1990s on. Quite a number of things have changed since then, though, and this is not just about sheer numbers. Equally important is the fact that today the biggest clients of private military services are the world's richest, most advanced states. So employment of contractors in combat is no longer about “state failure” as it was in the 1990s when African states lacking any sort of internal props or any means of substantial control over their territories hired mercenary forces to fight insurgents.

The use of contractors in war can be considered as the privatization of (certain) military functions. This is why the phenomenon is usually taken to challenge what is the most basic feature of the modern state for many: the monopoly of the legitimate use of violence. Most scholars recognize that this monopoly has never been complete and that modern states have indeed delegated the use of force to private agents in this way or some other. Some argue, for instance, that the history of military contracting in the United States dates back to the Civil War. Nonetheless, it is crucial to see that what we have been witnessing for the last two decades is a break from the past. There has been a dramatic change in the magnitude of military contracting:

Private contractors are now heavily employed in Afghanistan and Iraq. They are training local security forces (army and police), providing logistics support for the US military, escorting convoys, securing headquarters, and guarding top-level officials.

In what follows, I will review five of the most prominent studies on private contractors, which will hopefully help us reach an understanding of the current state of the field.