Complex sentence: In 1877 the fort became the headquarters for the newly created military district, the District of the Yellowstone (a subunitof the Department of Dakota) commanded by Miles who was promoted to brigadier general in September the same year.
Split into simpler sentences: In 1877 the fort became the headquarters for the newly created military district, the District of the Yellowstone (a subunit of the Department of Dakota), commanded by Miles. He was promoted to brigadier general in September the same year.