European nations and the United States share a common concern about their citizens going to the Middle East to fight for ISIS (the Islamic State of Iraq and Syria). Previous Ethical Musings' posts have addressed the problem of ISIS (cf. What to do about ISIS and ISIS). Contrary to inflammatory rhetoric, the numbers of Westerners fighting for ISIS are relatively small and only a minority of those will want to engage in terrorism upon return to their native land.
Unfortunately, the parallel phenomenon of Westerners going to Iraq, Syria, or another place in the Middle East to fight against ISIS has received little notice. As in the case of those supporting ISIS, numbers are small. Many of them espouse a form of Christian fundamentalism; most have fought in the armed forces of the US or another nation in Iraq or Afghanistan. As with their counterparts who fight for ISIS, some small number of these individuals will engage in criminal behavior when they return to their native land.
Westerners fighting for or against ISIS are indicative of two basic, widely ignored problems, both of which are more troubling than the issues I've seen addressed.
First, these fighters, regardless of the side for which they fight and the rhetoric they use to justify their actions, are marginalized individuals who are misfits in a peaceful society. They reject the possibility of peaceful progress toward a more just world. Among those who are military veterans, many are adrenalin junkies who miss the thrill of action to which they became addicted in the military. In different eras, Western misfits have gone to Russia, China, Africa, and elsewhere and fought for causes they deemed important. Instead of reactively worrying about returnees' potential to cause problems, nation states should proactively work to integrate into the economic and political mainstream those who inhabit the margins.
Second and more broadly, the trickle of mercenaries represents an erosion of the norm, supported by the Just War tradition and international law, that only nation states wage war. When an individual goes abroad to fight other than as part of that the armed forces of her/his native state, then that individual takes upon himself the right to make war and abrogates the larger community's responsibility for waging war.
When government privatizes its coercive power (e.g., by contracting with private companies to operate prisons or authorizing private security guards to arrest people and to use deadly force), government tacitly endorses the principle that it should not have a monopoly on the legitimate use of coercion. Mercenaries represent a step further down a slippery slope that culminates in a society in which might makes right and some will try to grab as much power as they can.