Mercenaries have existed for thousands of years but the earliest written record of them dates back to the 7th century BC, when Greek hoplites sought employment as bodyguards serving Pharaohs and Greek tyrants alike. As time wore on, mercenaries became a great way to bolster troop numbers during times of war, and oft added a sheen of experience and professionalism to the citizen and levy armies of the day. For an agreed fee the state could employ a veteran company, men who would stand fast against (almost) everyone - just as long as the silver continued to flow into their pockets.
