Complex sentence: He saw action at the Battle of Alma (20 September 1854) and the Battle of Balaklava (25 September 1854 onwards) and his actions saw his promotion to full Major in December 1854.
Split into simpler sentences: He saw action at the Battle of Alma (20 September 1854) and the Battle of Balaklava (25 September 1854 onwards). His actions here saw his promotion to full Major in December 1854.