Complex sentence: He fought in the Battle of Britain in 1940, and September 1941 was seconded to the Royal Australian Air Force to command No.453 Squadron RAAF, a fighter squadron then based in Singapore.
Split into simpler sentences: He fought in the Battle of Britain in 1940. In September 1941 he was seconded to the Royal Australian Air Force to command No.453 Squadron RAAF, a fighter squadron then based in Singapore.