This seh-ideiot has no knowledge of the history of the US military. Except for the emergencies of WWI and WWII, our military has always been composed of volunteers. The term mercenary, by the way, usually applied to armies for hire. She doesn't know that she is implying that a troops might sell out to a higher bidder. Fact is that, by and large, every one who has served in the military has come out a better person than he went in. Just compare them with their classmates, a significanr percentage of whom are in jail.
Mercenaries were in abundance during the Revolution (von Steuben, Polaski, Lafayette, J. P. Jones). A French mercenary was among the defenders of the Alamo (but he stole away before the fort fell). Several European mercenaries served on both sides during the War Between the States.
In WWII, the Flying Tigers were probably the most famous mercenaries, outside of the French Foreign Legion (which fought on both sides).
Biblically, David served as a mercenary to the Philistines before he became king.
"Mercenary" is not a perjorative. Indeed, mercenaries have often played pivotal roles in fighting for freedom. The most famous example might be Count Carl Gustav von Rosen, who flew gratis for the Biafrans during their abortive secession from Nigeria.