Complex sentence: In the fall of 1878 somewhere beyond the North Platte River after crossing into Nebraska the Cheyenne held council and it was discovered that 34 of the original 297 were missing, most had been killed but a few had decided to take other paths to the north.
Split into simpler sentences: In the fall of 1878 beyond the North Platte River, after crossing into Nebraska, the Cheyenne held council. They realized that 34 of the original 297 were missing; most had been killed but a few had decided to take other paths to the north.