Complex sentence: Due to the high casualty rate among its officers, Callis led the regiment at the Battle of South Mountain, Antietam and several other engagements and was promoted to Major on January 5,1863.
Split into simpler sentences: Due to the high casualty rate among its officers, Callis led the regiment at the Battle of South Mountain, Antietam and several other engagements. He was promoted to Major on January 5,1863.