Complex sentence: During the Second World War he served in the Royal Navy, and made his film debut in 1951, when he appeared in a major co-star role in the MGM film, Never So Few, starring Frank Sinatra and Gina Lollobrigida.
Split into simpler sentences: During the Second World War he served in the Royal Navy, and made his film debut in 1951. He appeared in a major co-star role in the MGM film, Never So Few, starring Frank Sinatra and Gina Lollobrigida.