Complex sentence: Thus he was happy to relinquish OB West in that month to Field Marshal Gerd von Rundstedt whilst he retained command of Army Group B, a post he would keep until the army group's final dissolution in April 1945.
Split into simpler sentences: Thus he was happy to relinquish OB West in that month to Field Marshal Gerd von Rundstedt. He retained command of Army Group B, a post he would keep until the army group's final dissolution in April 1945.