Complex sentence: Graf, a pre-war football player and glider pilot, joined the "Luftwaffe" in 1935 and initially selected for transport aviation was posted to "Jagdgeschwader 51" (JG 51--51st Fighter Wing) in May 1939.
Split into simpler sentences: Graf, a pre-war football player and glider pilot, joined the "Luftwaffe" in 1935. He was initially selected for transport aviation and was posted to "Jagdgeschwader 51" (JG 51--51st Fighter Wing) in May 1939.