Complex sentence: At the outbreak of the Civil War, he entered the Union Army as major of the 1st New Mexico Infantry Regiment, was later promoted to lieutenant colonel and took part in the Battle of Valverde in 1862.
Split into simpler sentences: At the outbreak of the Civil War, he entered the Union Army as major of the 1st New Mexico Infantry Regiment. He was later promoted to lieutenant colonel and took part in the Battle of Valverde in 1862.