Complex sentence: The region that became Glacier National Park was first inhabited by Native Americans and upon the arrival of European explorers, was dominated by the Blackfeet in the east and the Flathead in the western regions.
Split into simpler sentences: The region that became Glacier National Park was first inhabited by Native Americans. Upon the arrival of European explorers, it was dominated by the Blackfeet in the east and the Flathead in the western regions.