Mercenaries get a bad rap. The very word has become so anathematized that it is no longer used by those it describes, practitioners of one of the world’s oldest professions. Nowadays they prefer to be called “security contractors” and their employers prefer to be known as private military or security companies. This is an understandable if not entirely logical consequence of the state monopolization of warfare, which began in the late 18th century when governments became strong enough to conscript their own citizens to fight rather than rely on hired “free lances.” The French Revolutionary and Napoleonic Wars seemed to confirm that citizen armies were superior to the traditional mix of aristocrats and mercenaries employed by the ancien régimes, and before long almost everyone was emulating the French example. Along the way there arose the widespread belief that the use of citizen-soldiers was superior not only practically but also morally; there was something distasteful, even unethical, about hiring a professional soldier, often a foreigner, to fight on one’s behalf. Much better, leaders assumed, to force their own civilians to fight upon pain of punishment. This mindset has now become so deeply entrenched that it is easy to ignore the long and distinguished history of mercenaries, and their legitimate uses down to the present day.

As Peter W. Singer points out in his invaluable book, Corporate Warriors: The Rise of the Privatized Military Industry (2003), “Hiring outsiders to fight your battles is as old as war itself. Nearly every past empire, from the ancient Egyptians to the Victorian British, contracted foreign troops in some form or another.” The Greek city-states that founded Western civilization were heavily reliant on specialized units of mercenaries such as Cretan slingers and Thessalian cavalry to supplement their native hoplites. One of the great classics of literature, Xenophon’s Anabasis, chronicles the journey of 10,000 Greek mercenaries through what is today Iraq after participating in a Persian civil war. By the end of Alexander the Great’s stunning campaign of conquest, his army was made up primarily of foreigners, not Macedonians. Hannibal, likewise, scored his great victories against Rome in the Second Punic War with an army of hired hands. And although the Roman Empire by the end became overly reliant on unassimilated “barbarians” for protection, it thrived for hundreds of years by enlisting foreigners as auxiliaries to its legions.

The tradition continued into the Middle Ages and the Renaissance, when Italian mercenaries, organized into “companies” and hired through the condotta (contract) system, pioneered the very concept of the corporation. Some of the most feared soldiers of the period were Swiss infantrymen, who were hired in 1502 to protect the Pope and are still on the job today....