Complex sentence: He was subsequently promoted to the lieutenant-colonelcy of the 10th Regiment of Foot and performed the duty of commanding officer of the 10th upwards of twelve years, keeping the regiment in a high state of discipline and efficiency.
Split into simpler sentences: He was subsequently promoted to the lieutenant-colonelcy of the 10th Regiment of Foot. He performed the duty of commanding officer of the 10th upwards of twelve years, and preserved the regiment in a high state of discipline and efficiency.