The outbreak of World War II in 1939 interrupted his last year of studies. All theatre halls and concert-halls were closed. He managed to get a job as principal horn with the RAF Central Band which were in Uxbridge in north-west London. Dennis was always a very friendly person who got on well with everybody. The older players in the band might not have liked a 17-year-old boy to have a principal’s job, but everybody realized he had exceptional skills. He played in concerts whenever possible. There were lunch-hour concerts in the National Gallery during the war, and he often played in these. He started to become famous. He met Benjamin Britten and he persuaded him to compose his "Serenade for Tenor, Horn and Strings". He performed it in 1943 with the tenor Peter Pears and they recorded it for Decca in 1944. He toured the USA with the RAF Orchestra. Several conductors heard him and wanted Brain to join their orchestra.