Complex sentence: Harrison had originally arrested Rhea but allowed him to resign instead, he then placed Lt. Philip Ostander (one of the two lieutenants who had relieved Rhea) in command of the fort.
Split into simpler sentences: Harrison had originally arrested Rhea but allowed him to resign instead. He then placed Lieutenant Philip Ostander (one of the two lieutenants who had relieved Rhea) in command of the fort.