Complex sentence: He was commissioned into the Royal Tank Regiment in 1963 as a second lieutenant, but his career there did not last long; he left military service and moved to France, where he worked in, and then owned, a restaurant.
Split into simpler sentences: He was commissioned into the Royal Tank Regiment in 1963 as a second lieutenant, but his career there did not last long. He left and moved to France, where he worked in, and then owned, a restaurant.