National security has always relied upon industry, but the current situation is unprecedented. In 1991, the uniform-to-contractor ratio of personnel deployed to the Persian Gulf was fifty to one. Reductions in American and NATO force strengths following the collapse of the Soviet Union were predicated on a belief that in future conflicts, reductions in manpower could be mitigated by increased reliance on technology. With extreme manpower requirements in Iraq, and the subsequent surge of forces in Afghanistan, the current uniform-to-contractor ratio is one-to-one.
In Tim Spicer's book, he advocated a system whereby legitimate PMCs would employ military veterans to assist legitimate governments in national security challenges. Rather than providing combat forces, Spicer envisioned PMCs assisting legitimate governments by providing training, logistical, aviation, and other types of support to host nation combat forces. Operating independently of national military forces, Spicer proposed that PMCs could volunteer for regulation by the United Nations. For the most part, Spicer's vision has failed to materialize.
What has happened instead is that "private security companies", as well as more long-standing defense contractors, have been retained by national militaries - primarily by the Department of Defense - to augment the uniformed personnel. This phenomenon occurs in both domestic and deployed units. There are few aspects of national security that aren't touched in one way or another by contractors and/or PSCs. Many contractors even carry weapons, providing security for bases, convoys, and high value personnel.
Thus far, I've sourced some books, but I haven't sourced any articles. Here's what I have so far.
I know that the DoD has done some research on the long-term impact of employing PSC/PMC personnel, but I'm not sure how much of that research is in the public domain. I may be able to find some information at some of my old standby sources: Small Wars Journal, the Foreign Military Studies Office, Global Security, and the RAND Corporation.