You said someone hired... What about Americans through private companies. What does "assist in fighting a war" cover?
"In February, Blackwater, through a subcontractor, began hiring former combat personnel in Chile, offering up to $4,000 a month to guard oil wells in Iraq, the company official said. Some of the Chileans worked for former Chilean dictator Augusto Pinochet, the spokesman confirmed.
You got this thing mixed up.
I don't think they're any more a mercenary than is a Mexican security guard watching a K-Mart in Kansas.
A contract was let for a specific security service and this company was able to provide the specifications of the contract.
The Hessians in the Revolutionary War were mercenaries. A hired group of foreign fighters.