Complex sentence: Colonel Benjamin Bonneville was in command which entered the frontier going west from Fort Fillmore, New Mexico, across the desert where they engaged in two small but significant battles in the Black Range and along the Gila River near present day Safford, Arizona.
Split into simpler sentences: Colonel Benjamin Bonneville was in command of the party which entered the frontier from Fort Fillmore, New Mexico. The first was fought in the Black Range and the second along the Gila River near present day Safford, Arizona.