A mercenary is a soldier who fights for money, regardless of ideological, national or political considerations. When addressed to a professional soldier in a regular national army, the term is normally used as an insult or epithet.
In the late Roman Empire, Roman citizens weren't ready for military service and the empire often contracted whole bands of barbarians either within the legions or as autonomous foederati. The barbarians were Romanized and surviving veterans were established in areas requiring population.
See also Anabasis
In Italy, the condottiero was a military chief offering his troops, the condottieri, to city-states.
During the ages of the Taifa kingdoms of the Iberian peninsula, Christian knights like the Cid coud fight for some Muslim ruler against his Christian or Muslim enemies.
The Almogavares fought for Aragon but in their expedition to Orient, they followed Roger de Flor in the service of the Byzantine Empire.
See also: Bertrand Duguesclin, White Company, Scottish clan.
It was then that the European landsknechts, colorful mercenaries with a redoubtable reputation, took over the Swiss forces' legacy and became the most formidable force of the late 15th and throughout the 16th century, being hired by all the powers in Europe and often fighting at opposite sides.
St Thomas More in his Utopia advocated the use of mercenaries in preference to citizens.
In the 20th century, mercenaries have been mostly involved in conflicts on the continent of Africa. There have been a number of unsavory incidents in the brushfire wars of Africa, some involving recruitment of naive European and American men "looking for adventure" and thrusting them into combat situations where they would not survive to get paid.
Many of the adventurers in Africa who have been described as mercenaries were in fact ideologically motivated to support particular governments, and would not fight "for the highest bidder."
One particularly notorious mercenary was Mike Hoare.
The French Foreign Legion is not a mercenary outfit as it is deployed and fights in the French national interest, but many of the persons who choose to join it could be described as mercenaries.
It is known that mercenaries have been hired to fight in the conflicts in former Yugoslavia. Many of these were ex-Eastern Bloc soldiers who had no employment opportunities after the fall of the Soviet Union.
The largest mercenary outfit engaged in actual warfare, Executive Outcomes, consists largely of former members of the then white-dominated South African miltiary. EO operates in battalion strength with organic helicopter gunship support.
It could be argued that paramilitary forces under private control are functionally mercenaries instead of security guards. However, national governments reserve the right to strictly regulate the number, nature and armaments of such private forces.
Mercenaries are often considered illegal combatants according to the laws of war, as they are not under the command of a commissioned officer of a national government. Captured mercenaries are often treated as common criminals and can expect execution, sometimes by torture.
Under United States law (the "Neutrality Act"), an American citizen who participates in an armed conflict to which the United States is neutral may be subject to criminal penalties.
The legal status of civilian contractors, particularly logistics staff and equipment technicians, serving with miltary forces is unclear. It has been argued that they are functionally mercenaries because they are assisting in active military operations even though they are not firing weapons at the enemy. This is not a hypothetical concern. In the United States Navy for example, the typical aircraft carrier has between 50 and 100 civilians aboard acting as technicians, manufacturer's representatives, etc.
See also privateer, Letter of marque
Like piracy, the mercenary ethos resonates with idealized adventure, mystery and danger. A good example is the book and movie Dogs of War which goes into some detail about an actual if fictionalized mercenary operation in Africa in the 1960s.
A magazine ostensibly written for mercenary soldiers is Soldier of Fortune.
In science fiction, the well-known author Jerry Pournelle has written several books about science-fiction mercenaries kown as Falkenberg's Legion. Also, author David Drake has written a number of books about the fictional hovercraft armored regiment Hammer's Slammers. Both series of books are brutal in their portrayal of complex low-intensity warfare despite technological advances.
See also The Magnificent Seven, ronin, yojimbo, Gurkha.Mercenaries in European History
Mercenaries in the Classic era
Carthage contracted Balearic Islands shepherds as slingshooters during the Punic wars against Rome.Mercenaries in Medieval warfare
Byzantine Emperors followed the Roman practise and contracted foreigners especially for their personal corps guard. They were chosen among war-prone peoples. Since they didn't have links to the Greeks, they were expected to be ready to suffocate rebellions. Varangians and Anglo-Saxons were elected for this service.Mercenaries in the Modern Age
Swiss mercenaries were sought after during the latter half of the 15th century as being an effective fighting force, until their somewhat rigid battle formations became vulnerable to arquebuseses and artillery being developed at about that period.Mercenaries in Africa
Mercenary Operations
Mercenaries and the Laws of War
Mercenaries in Popular Culture