Complex sentence: Fearing an attack the next day, Lynde abandoned Fort Fillmore (after destroying ammunition and supplies in the citadel) and retreated northeast towards Fort Stanton across the dry Organ Mountains.
Split into simpler sentences: Fearing an attack the next day, Lynde abandoned Fort Fillmore after destroying the ammunition and supplies in the citadel. He retreated northeast towards Fort Stanton across the dry Organ Mountains.