Mercenaries are soldiers who fight for money, rather than for their country, political/religious beliefs, or other benefits.

Mercenaries, DSS, Diplomatic Security Service

Mercenaries are a very old phenomenon. They are mentioned in the Old Testament, and King David had a bodyguard of Cretan and Philistine mercenaries. They are also mentioned in the Iliad, specifically the Myrmidons who fought in the Trojan War (12th century BC).