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Mercenaries are a very old phenomenon. They are mentioned in the Old Testament, and King David had a bodyguard of Cretan and Philistine mercenaries. The Carthaginians often used mercenaries. The so-called Free Companies in the Middle Ages were mercenary companies which consisted of rogue knights and archers. The Japanese ronin were mercenary samurai.
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 now not a mercenary force because all of its members Catholic, and they are prepared to fight for not their country but 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 sincw they are loyal to the country they serve. However, various private military contractors are considered mercenaries.