Complex sentence: The history of the 21st Battalion, which the PWOR perpetuates, is far too long to relate here, however, it should be mentioned that the unit earned eighteen Battle Honours were won in three years of frontline service.
Split into simpler sentences: The history of the 21st Battalion, which the PWOR perpetuates, is far too long to relate here. Suffice to say, eighteen Battle Honours were won in three years of frontline service.