Complex sentence: During the Civil War, he sold his journal and entered the Union Army late in 1862 as a major of the 15th Illinois Cavalry, commanded the 4th U.S.C. Cavalry as colonel, and left the Army in 1865 as a brevet brigadier general.
Split into simpler sentences: During the Civil War, he sold his journal and entered the Union Army late in 1862. He was commissioned as a major of the 15th Illinois Cavalry, commanded the 4th U.S.C. Cavalry as colonel, and left the Army in 1865 as a brevet brigadier general.