Final answer:
The mercenary soldiers hired by Britain during the Revolutionary War were called Hessians, and their involvement was one of the grievances in the Declaration of Independence.
Step-by-step explanation:
The mercenary soldiers Britain hired to fight in the Revolutionary War were known as Hessians. These mercenaries were employed by the British at key battles, such as the engagements at Trenton and Princeton. The use of these German troops was contentious and even listed as a grievance in the Declaration of Independence, due to the requirement that colonists house them. Despite the impression held by British officers like Major John Pitcairn that a forceful campaign could swiftly end the American rebellion, the conflict proved to be more protracted and complex than they had anticipated.