Complex sentence: Upon the outbreak of the Civil War, Walker served as a colonel of the 4th Regiment, Arkansas State Troops; he was captured at Oak Hills, Missouri in 1861 and held as a prisoner of war for two years.
Split into simpler sentences: Upon the outbreak of the Civil War, Walker served as a colonel of the 4th Regiment, Arkansas State Troops. Captured at Oak Hills, Missouri in 1861, he was held as a prisoner of war for two years.