Complex sentence: Shortly after the U.S. Civil War began, he attempted to enlist into the Confederacy when he was 15 years old who stood 6 feet in height, his officers discovered his true age and was quickly discharged from military service.
Split into simpler sentences: Shortly after the U.S. Civil War began, he attempted to enlist into the Confederacy when he was 15 years old. Although he stood 6 feet in height, his officers discovered his true age and he was quickly discharged from military service.