Complex sentence: He was was promoted to Lt. Colonel and served as the executive officer of the 24th Mississippi under the command of Colonel William F. Dowd which was a part of Major General Walthall's Brigade.
Split into simpler sentences: He was was promoted to Lt. Colonel and was transfrred to the 24th Mississippi. He served as the executive officer of the 24th Mississippi under the command of Colonel William F. Dowd which was a part of Major General Walthall's Brigade.