hired professional soldier who fights for any state or nation without regard to political interests or issues. From the earliest days of organized warfare until the development of political standing armies in the mid-17th century, governments frequently supplemented their military forces with mercenaries.

Employment of mercenaries could be politically dangerous as well as expensive, as in the case of the early 14th-century almogaváres, Spanish frontiersmen hired by the Byzantine Empire to fight the Turks. After helping defeat the enemy, the almogaváres turned on their patrons and attacked the Byzantine town of Magnesia (modern Alaşehir, Tur.). After the assassination of their leader they spent two years ravaging Thrace and then moved on to Macedonia.

Following the Hundred Years' War (1337–1453), Europe was overrun with thousands of men who had been trained for nothing but fighting. During the 15th century “free companies” of Swiss, Italian, and German soldiers sold their services to various princes and dukes. These hired soldiers, often greedy, brutal, and undisciplined, were capable of deserting on the eve of battle, betraying their patrons, and plundering civilians. Much of their mutinous behaviour was the result of their employer's unwillingness or inability to pay for their services. When rigid discipline, sustained by prompt payment, was enforced (as in the army of Maurice of Nassau), mercenaries could prove to be effective soldiers. Swiss soldiers were hired out on a large scale all over Europe by their own cantonal governments and enjoyed a high reputation. In 18th-century France the Swiss regiments were elite formations in the regular army.

Since the late 18th century, however, mercenaries have been, for the most part, individual soldiers of fortune. Since World War II they have won some prominence for their exploits in certain Third World countries, especially in Africa, where they were hired both by government and by antigovernment groups.