Complex sentence: He served on campaigns on the North West Frontier and in the First World War, at first in France on the Western Front and then in the Sinai and Palestine Campaign, during which he commanded the 9th (Secunderabad) Cavalry Brigade and the 11th Cavalry Brigade.
Split into simpler sentences: He served on campaigns on the North West Frontier and in the First World War. During which he commanded the 9th (Secunderabad) Cavalry Brigade and the 11th Cavalry Brigade.