Complex sentence: Lahm was chosen to be his chief of staff, and remained so until after the Battle of Chateau-Thierry, when Foulois voluntarily turned over his command to Brig.
Split into simpler sentences: Lahm was chosen to be his chief of staff. He remained on staff until after the Battle of Chateau-Thierry, when Foulois voluntarily turned over his command to Brig.