Ferdinand II of Habsburg, in order to fight against Sweden, in 1631, employed the mercenary captain Albrecht Von Wallenstein to lead the monarchy's army. The well known and excellent "condottiere", did beat Sweden and betrayed the Emperor's trust by withdrawing the army, rather than occupying Saxony. I found interesting the connection with Machiavelli's masterpiece, written one century before this episode.
According to Machiavelli mercenaries were the cause of Italy's instability : "Mercenaries forces are useless and dangerous. [..] the mercenaries' captains are either excellent men or not: if they are, they will always run over their own greatness; if they are not virtuosi, they will ruin the principate's army." (The Prince pp.43-4)"