Complex sentence: Jagdkorps (1st Fighter Corps) had long argued for a repeat of intruder operations and first pressed for their resumption in December 1943 which were rejected by the "Oberkommando der Luftwaffe" (High Command).
Split into simpler sentences: Jagdkorps (1st Fighter Corps) had long argued for a repeat of intruder operations and first pressed for their resumption in December 1943. His proposals were rejected by the "Oberkommando der Luftwaffe" (High Command).