Complex sentence: He once again commanded 2nd Battalion at Hazebrouck in April 1918, where which it took such severe casualties that it saw no further action, after which, still an acting lieutenant-colonel, he commanded a corps infantry school in October 1918.
Split into simpler sentences: He once again commanded 2nd Battalion at Hazebrouck in April 1918, where which it took such severe casualties that it saw no further action. Still an acting lieutenant-colonel, he then commanded a corps infantry school in October 1918.