Complex sentence: He assumed command of the 118th Pennsylvania Regiment part way through the war which participated in 39 recorded battles including Seven Pines, Fredericksburg, Shepherdstown, Five Forks, and Gettysburg.
Split into simpler sentences: He assumed command of the 118th Pennsylvania Regiment part way through the war. That regiment participated in 39 recorded battles including Seven Pines, Fredericksburg, Shepherdstown, Five Forks, and Gettysburg.