Complex sentence: In the year of 1837, before the outbreak of the Civil War, he was sold as a slave to Joseph Emory Davis--whose brother, Jefferson Davis, later became the President of the Confederate States of America.
Split into simpler sentences: In the year of 1837, before the outbreak of the Civil War, he was sold as a slave to Joseph Emory Davis. Davis's brother, Jefferson Davis, later became the President of the Confederate States of America.