But history has some very pointed observations about the use of mercenaries, observations that are pertinent now, today, this very moment. Mercenaries have existed for a long time, and being a hireling soldier, rather than a conscript, has even been respectable in some times and places (ancient Greece, for instance--Xenophon was accompanying 10,000 Greek mercs in Cyrus's army when they were trapped in enemy territory after their employer was killed in the battle. Hiring out as a soldier was quite respectable for Greeks of that day, as it was for Swiss soldiers later.) There's nothing new about mercenaries, or about the dangers they pose to their employers.
All military forces are dangerous. If they aren't dangerous, they're useless (they can be useless *and* dangerous, but that's another topic.) Once you give a lot of young people lethal weapons, and teach them to use those weapons, you've got a social element that can damage your own society as well as those of your enemies. And among the group will be a few whose personal psychology thrives on adrenalin--on danger--and those few can easily be a problem.
The safest military is a regular army (using the term generally--it's true of any branch) in a society that is committed to a rule of civilian law, with the members of that military representing a range of opinions and types found there. Such a military, composed of citizens loyal to the ideals of that society--to the rule of law, to the belief that civilians are competent and should be in charge--is less likely to attempt to seize power than any other. The larger the fraction of the population that serves, or has served, in such a military, and the more thoroughly the military culture is imbued with the ideals of the civilians, the safer it is. (Still not "safe"...but safer.)
The most dangerous, throughout history, is the mercenary army whose members include a mix of soldiers from many other armies (and thus no unifying values aside from those necessary for war), whose loyalty is to its own commander (even if not called a commander--a CEO, a boss) when that commander's ideals are not in concert with those of the hiring party. Mercenaries thrive on war, and go hungry in peace. It is to their advantage for wars to exist (the same is not true of national armies, which can thrive in peace to the extent their governments keep paying the bills. Politicians may fret that the military is just sitting around costing them money, but the professional national military is commonly quite happy to have no war.) It is to mercs' advantage for there to be "incidents" and "insurgencies"--it justifies their pay, and their pay needs justification. Because they often have to go from war to war to stay solvent, mercs tend to have more combat experience than most regular soldiers--more wars in more places fighting different enemies. They know more tricks of the trade.
For mercs to stay bought, they need to be paid well; hirers who run out of money (and this has happened at various times throughout history) may find that their mercs are now someone else's mercs, or at the least their mercs have decided to take their pay out in looting the city/state/country that hired them. History shows that it's unwise to assume mercs retain loyalty their place of birth, or any real loyalty to their employers: their loyalty is to their group's leader, and that leader's standards determine the group's standards. Even mercs who are paid fully and on time may decide they want more and have the power to take it; mercs who are more experienced and better equipped than the official indigenous military force have done just that. Military coups can be made--have been made--by mercenary commanders (see the history of the Italian city-states and the condittieri.)
I've written about mercenaries in many of my books, because (as a veteran myself and also a student of history) the whole range of military ethos fascinates me. I've chosen to write mostly about "good" mercs, but have also (particularly in the Paksenarrion books) shown some "bad" mercs as well. Mercenary soldiering, as opposed to regular-military soldiering, attracts people who like the excitement of war but dislike the discipline, the restrictions, of the regular military. They're impatient with restraint; they're uninterested in the higher concerns (strategy and grand strategy) and just want to get it done. Mercenary companies traditionally do not care as much about some background characteristics and history as the regular military. Our military, for instance, is much less willing to take someone with a criminal history--they anticipate discipline problems down the line. They're extremely mission oriented, at the operational and tactical level, leading to (often, not always) very efficient, effective action directed at that one assigned mission. They're less likely to recognize, respect, or want to care about the long-term strategic aims. This means they need to be under some kind of control so the employer's strategic aims are not frustrated by the mercs' short-term and narrow viewpoint. And right there comes the first big problem.
Mercs do not play well with others. It is hard enough to impose a coherent strategy on multiple branches of one military, or allied regular militaries (ask Eisenhower!) but at least in this case governments can agree on who is the supreme commander, and a chain of command exists. Mercenaries often sit outside any regular chain of command...they are not accountable to commanders other than at the top--their commander to someone else--and that, as any military historian or veteran can tell you, creates chaos. They have their own values, their own way of doing things, and they're about as likely to cooperate with one another as a cat and a dog.
In the current conflicts in Iraq and Afghanistan, there are many, many merc groups, each with its own individual chain of command, each with its own aims, its own missions assigned not by the US military command but by someone else--the State Department, the Oval Office, another contract who hired that particular merc group to provide security for its personnel. There is no coordination, no clear chain of command that includes the merc units, so that, for instance, a US officer commanding a unit in the field knows who is commanding the nearest merc unit and what they're up to. US soldiers have reported multiple instances in which mercs have behaved in ways that made life worse for the regular military personnel...mercs got their own job done (protected a diplomat, let's say) but screwed up other people's jobs in the process.
Employers can--and should--establish a clear, unified chain of command if they wish to employ mercs, especially in a mixed setting (both regular military and mercenaries operating in the same territory)--the mercs should be kept subordinate to their regular military; the rules governing interaction between mercs and regularly military personnel at all levels should be spelled out and agreed on.
Next rock: more on the problems of using mercenaries