As I have noted previously, trying to apply International Humanitarian law (IHL) to private contractors is often extremely difficult.
There is, of course, much precedent for the presence on the battlefield of individuals who are not formally members of the belligerents' armed forces. The 1949 Geneva Convention on Prisoners of War (POW) granted POW treatment to civilians who "accompany the armed forces without being members thereof, such as civilian members of military aircraft crews, war correspondents, supply contractors, and members of labor units or of services responsible for the welfare of the armed forces."
Civilians enjoyed protection against direct attack; however, it was well accepted by this time that if they took up arms they rendered themselves targetable. In a memorable event involving such individuals, over one-half of the American defenders at Wake Island were civilian contractors building a U.S. naval base when the Japanese attacked in December 1941.
Still, despite the fact that the latest wave of private contractors is now at least twenty years old, to give a conservative estimate, trying to decide their status is still enormously contentious.
To understand just how contention see this article by Michael Schmitt, professor of public international law at Durham University Law School.
Schmitt starts by noting that in 2003, the International Committee of the Red Cross (ICRC), in cooperation with the T.M.C. Asser Institute, launched a major research effort to explore the concept of "direct participation by civilians in hostilities" (DPH Project). The goal was to provide greater clarity regarding the international humanitarian law (IHL) governing the loss of protection from attack when civilians involve themselves in armed conflict.
Although the planned output of the project was a consensus document, the proceedings proved highly contentious. As a result, the final product contains the express caveat that it is "an expression solely of the ICRC's views".
Aspects of the draft circulated to the experts were so controversial that a significant number of them asked that their names be deleted as participants, lest inclusion be misinterpreted as support for the Interpretive Guidance's propositions. Eventually, the ICRC took the unusual step of publishing the Interpretive Guidance without identifying participants. Schmitt participated throughout the project, including presentation of one of the foundational papers around which discussion centered. He was also one of those who withdrew his name upon reviewing the final draft.
Although his article is not primarily on private contractors Schmitt notes:
At the outset of these conflicts, the activities and status of contractors were relatively unregulated in either law or policy. As a result of the public attention drawn by the scale of their presence and repeated incidents of misconduct, some states have endeavored to define the legal status of contractors and to create systems whereby they can be held accountable for abuses they commit. Additionally, states sending and those receiving contractors and civilian employees have negotiated status of forces agreements, which establish jurisdictional prerogatives; the agreement signed between the United States and Iraq in November 2008 is especially notable. States have also begun to adopt common "best practices" regarding private military companies, as exemplified by the ICRC/Swiss government sponsored 2009 Montreux ocument.