Merces (lat.) = ware, can also be money (metal as a ware), hence mercenary.
Soldo (ital.) = money paid for service rendered, hence soldier.
In the end it makes no difference: Both are people in armed service professionally fighting for their subsistence.
That mercenaries can have the same loyalty as soldiers is best shown by the Swiss mercenaries of the Bishop of Rome (aka The Pope) during the sack of Rome, or of the late King Louis XVI - in defence of him his Swiss guards were massacred to the last man.
Now, Charmion, what does the army lawyer say?