Complex sentence: The Arapaho left before 1860 when the area was settled by people of European descent and by 1878 the northern Arapaho were forced into a reservation at Wind River Indian Reservation.
Split into simpler sentences: The Arapaho left before 1860 when the area was settled by people of European descent. By 1878, the northern Arapaho were forced into a reservation at Wind River Indian Reservation.