Complex sentence: Force and Green had left the fort, on horseback, to investigate the previous night's disturbances and were several miles east of Fort Blue Mounds when they came across Black Hawk's band, guided to the fort by sympathetic Ho-Chunk.
Split into simpler sentences: Force and Green had left the fort, on horseback, to investigate the previous night's disturbances. They were several miles east of Fort Blue Mounds when they came across Black Hawk's band, guided to the fort by sympathetic Ho-Chunk.