Complex sentence: When the Second World War broke out, Rees returned to the United Kingdom from the Bahamas and once again joined the RAF, he relinquished his rank of group captain in January 1941 at his own request and was granted the rank of wing commander.
Split into simpler sentences: When the Second World War broke out, Rees returned to the United Kingdom from the Bahamas and once again joined the RAF. He relinquished his rank of group captain in January 1941 at his own request and was granted the rank of wing commander.