An editor thinks that this article may not be neutral. (May 2019)
This article does not have any sources. (May 2019)
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).
For many years in the Middle Ages, soldiers from Switzerland fought battles for whoever paid them. The popes would hire soldiers from Switzerland to fight for them. The Swiss Guard is all that is left of Switzerland's mercenary tradition, but it is no longer a mercenary force because all its members are Catholic, and they are prepared to fight not for their country but for their beliefs.
Soldiers of professional volunteer armies, like the US Army, are not considered as mercenaries since they fight for their country and fatherland. Likewise, foreign volunteers in standing armies, such as French Foreign Legion, are not considered as mercenaries since they are loyal to the country that they serve. However, various private military contractors are considered as mercenaries.