Complex sentence: After he retired from active duty, Brown began a circular with information on opportunities for mercenary employment in Oman, where the Sultan Qaboos had recently overthrown his father and was battling a communist insurgency.
Split into simpler sentences: After he retired from active duty, Brown began a circular with information on opportunities for mercenary employment in Oman. There, the Sultan Qaboos had recently overthrown his father and was battling a communist insurgency.