Complex sentence: Messines, Terhand and Dadizeele fell on 29 September and by the next day, despite the captured ground becoming another slough of mud, all of the high ground around Ypres had been occupied by the Allies.
Split into simpler sentences: Messines, Terhand and Dadizeele fell on 29 September. By 30 September, despite the captured ground becoming another slough of mud, all of the high ground around Ypres had been occupied by the Allies.