Soldier hired by the army of another country or by a private army, or a Soldier that will fight for you for pay enough.
They can be called private security guards
The word for a soldier-for-hire is mercenary
German solder hired by the British=hessian
mercenary
the goverment hiered the soldiers to fight
A veteran or vet.
Patriot?
GI's
A Conquistador
A Valliant Knight?
mercenary