Complex sentence: In about the mid 1870's military campaigns against Indian tribes reduced the danger from roving bands of Indians on the Eastern Montana plains, which allowed buffalo hunters to roam over the prairie and kill buffalo in large numbers.
Split into simpler sentences: In about the mid 1870's military campaigns against Indian tribes reduced the danger from roving bands of Indians on the Eastern Montana plains. As the Indian bands decreased the "hide hunters" began to roam over the Eastern Montana prairie and kill buffalo in large numbers.