Complex sentence: After a brief sojourn in business, he served in the Royal Flying Corps and RNAS during the First World War and also rejoined the Royal Air Force in 1939 as a Squadron Leader.
Split into simpler sentences: After a brief sojourn in business, he served in the Royal Flying Corps and the Royal Naval Air Service during the First World War. He also rejoined the Royal Air Force in 1939 as a Squadron Leader.