Final answer:
George Rogers Clark was the head of the militia force defending the western frontier in the late 1770s, contributing significantly to the expansion of the United States' frontier.
Step-by-step explanation:
The head of the militia force defending the western frontier in the late 1770s was George Rogers Clark. He led a small force of frontiersmen through the western territories and played a significant role in extending American influence in the area, particularly through his capture of British forts in the Illinois country during the Revolutionary War. Clark's efforts were instrumental in expanding the United States' frontier and securing the Northwest Territory.