Complex sentence: It took the British over a year to develop their own mustard gas weapon (their only option was the Despretz--Niemann--Guthrie process), first using it in September 1918 during the breaking of the Hindenburg Line.
Split into simpler sentences: It took the British more than a year to develop their own mustard gas weapon. This was used first in September 1918 during the breaking of the Hindenburg Line.